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Space Telescope Science Institute
JAMES WEBB SPACE TELESCOPE MISSION
SCIENCE AND OPERATIONS CENTER
Overview of NIRSpec Calibration Activities
Revision -
Released: July 7, 2006
Operated by the Association of Universities for Research in Astronomy, Inc., for the National Aeronautics and Space Administration under
Contract NAS5-03127
Overview of NIRSpec Calibration Activities
JWST-STScI-000851, SM-12
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CM Foreword
This document is an STScI JWST Configuration Management-controlled document.
Changes to this document require prior approval of the STScI JWST CCB. Proposed
changes should be submitted to the JWST Office of Configuration Management.
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Overview of NIRSpec Calibration Activities
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Signature Page
Prepared by:
Electronic Signature
Charles D. Keyes
Principal Program Manager
STScI/PM
04/01/06
Reviewed by:
Electronic Signature
Knox Long
WBS Manager
JWST Flight Systems Support
STScI/JWST Mission Office
06/30/06
Approved by:
Electronic Signature
David Hunter
Project Manager
STScI/JWST Mission Office
07/06/06
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STScI JWST Document Change Record
Title: Overview of NIRCam Calibration Activities
STScI JWST CI No: JWST-STScI-000851
Change No./Date
Description of Change
JWST-STScI-CR-000789
Baseline document
Revision: Baseline (-)
Change Authorization/Release:
CCB 7/7/06
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Table of Contents
Space Telescope Science Institute...................................................................................................i
JAMES WEBB SPACE TELESCOPE MISSION..........................................................................i
SCIENCE AND OPERATIONS CENTER.....................................................................................i
Overview of NIRSpec Calibration Activities..................................................................................i
Revision -.........................................................................................................................................i
Released: July 7, 2006....................................................................................................................i
1 Introduction...................................................................................................................................1
1.1 Scope and Purpose of this Document....................................................................................1
1.2 Various Goals of Calibration.................................................................................................2
1.3 NIRSpec Instrument Purpose.................................................................................................3
1.4 NIRSpec Instrument Design..................................................................................................3
1.5 Supporting Documents and Presentations.............................................................................7
2 Sources of Calibration Information..............................................................................................1
2.1 Overview of Assembly, Integration and Testing...................................................................1
2.2 Overview of On-Orbit Commissioning.................................................................................5
2.3 Onboard Calibration Lamps...................................................................................................6
3 Assessment of Ground Calibration Needs....................................................................................1
3.1 Transmissive Optical Components (6)...................................................................................1
3.2 Dispersive Elements Gratings (7)..........................................................................................2
3.3 Non-dispersive Reflective Optical Elements (Mirrors) (14).................................................3
3.4 Opto-mechanical elements (grating wheel, e-focus mechanism, etc) (3)..............................3
3.5 Camera Systems (3 elements)................................................................................................4
3.6 Apertures (fixed and MSA) (5 fixed slits; IFU aperture; MSA)............................................4
3.7 Integral Field Unit..................................................................................................................5
3.8 Calibration Assembly.............................................................................................................6
3.9 Micro-Shutter Array (MSA)..................................................................................................7
3.10 Sensor Chip Assemblies (detectors) (2)...............................................................................8
3.11 Optical Performance/Alignment........................................................................................12
3.12 Geometric Characteristics and Location of Images...........................................................13
3.13 Target Acquisition.............................................................................................................14
3.14 Internal Calibration Channel..............................................................................................14
3.15 Mechanism Motion and Repeatability...............................................................................14
3.16 Spatial Uniformity and Photometric Calibrations..............................................................14
3.17 Spectroscopic and Wavelength Calibrations.....................................................................15
3.18 Integral Field Unit Calibration...........................................................................................17
4 Assessment of On-Orbit Calibration Needs..................................................................................1
4.2 Astrometric Calibration and Image Quality...........................................................................4
4.3 Flat-Field Calibration (Differential Photometric Calibration)...............................................6
4.4 Absolute Photometric Calibrations........................................................................................6
4.5 Wavelength Calibration.........................................................................................................8
4.6 Calibration of Stray and Scattered Light...............................................................................9
5 Draft Science Calibration Pipeline................................................................................................1
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5.1 Overview of Processing for Data from JWST Science Instruments......................................1
5.2 File Formats...........................................................................................................................2
5.3 Pipeline Structure...................................................................................................................3
5.4 A Draft Calibration Pipeline “calNIRSpec”..........................................................................3
5.5 Baseline Strategy for Supplying and Maintaining Reference Files.....................................12
5.6 Summary of Required Calibration Reference Files by Mode..............................................15
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List of Figures
Figure 1:
Figure 2:
Figure 3:
Figure 4:
Schematic diagram showing the layout of the NIRSpec.................................................5
Schematic layout of the NIRSpec slit mask overlaid on the detector array....................5
HST standard stars suitable for photometric calibration of NIRSpec;............................8
Draft outline of NIRSpec pipeline ..................................................................................6
List of Tables
Table 1: NIRSpec Optical Element, Bandpass, and Slit characteristics (all modes). ...................4
Table 2: NIRSpec SI characteristics (standard modes)...................................................................6
Table 3: NIRSpec-related Documents............................................................................................7
Table 4: Calibration Sources in NIRSpec.......................................................................................6
Table 5 : Draft Flow Chart for calNIRSpecA..................................................................................8
Table 6: Acquisition and Maintenance of Possible Reference Files ............................................15
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Introduction
This document will provide an overview of the activities needed to calibrate the JWST NearInfrared Spectrograph (NIRSpec) fully. Although many of these activities are of critical
importance and all are highly desirable, only those items derived from items in the “NearInfrared Spectrograph Functional Requirement Document” are to be considered as formal
requirements that need to be verified.
1.1
Scope and Purpose of this Document
The purpose of this document is to provide a comprehensive overview of the activities to be
performed on the ground at component and system level and in flight during verification and,
subsequently, in the science operations period that are required to calibrate NIRSpec, by
describing:
1. The measurements required characterizing the instrument completely and optimizing its
performance.
2. The corrections that need to be implemented in order to make images from NIRSpec
useful for scientific analysis.
3. The baseline plans for the procurement of the ancillary information required to correct
NIRSpec images during ground-based testing and on-orbit operations.
The approach utilized to evaluate the required activities starts with considering the following
questions relating to the requirements of the data processing pipeline, value-added scientific
analysis, and sound instrument development practices:
1. What references files are needed to support pipeline processing? What calibration data,
observations, and tests are needed to produce the information from which the calibration
reference files can be produced?
2. Are there any special datasets required to characterize the SI regardless of their relevance
to the creation of pipeline reference files?
3. Are there special datasets required for the support of more detailed scientific analysis that
extends beyond routine pipeline processing?
4. Lastly, are their any calibrations, special datasets, or important procedures that should be
obtained or utilized whose utility stems from important lessons learned in HST
instrument development and calibration?
Of course, the detailed nature of these calibration activities will evolve as the instrument design
matures and construction proceeds. Elaboration of the activities outlined here will ultimately
lead to the production of two deliverable items from the Instrument Development Team (IDT):
1. GSW-01: Science Instrument Ground Software Report (calibration algorithms and
documented I&T software)
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2. GSW-02: Calibration Reference Files (initial suite of calibration files, with
documentation concerning their format)
Although there are few formal requirements for the properties of the final, calibrated data
products from the NIRSpec, such requirements are implied by the stated performance goals
contained in the NIRSpec Functional Requirements Document. These are mentioned when
relevant.
It is anticipated that, at a later date, a comprehensive NIRSpec Calibration Plan (NIRSpec DRD
OPS-03) for the James Webb Space Telescope (JWST) mission will be prepared.
This, in turn, will serve as part of the input to the overall JWST Calibration Plan (SOC DRD OP09) to be prepared by STScI. The NIRSpec Calibration Plan will define in detail how the
instrument will be calibrated to accomplish the science objectives for NIRSpec described in the
JWST Science Requirements Document (JWST-RQMT-002258). The NIRSpec Calibration
Plan will implement the calibration requirements outlined in the NIRSpec Functional
Requirements Document (ESA–JWST–SPEC–002060) and follows the procedures outlined in
the NIRSpec Operations Concept Document (STScI-JWST-R-2003-0003). The NIRSpec
Calibration Plan shall support the calibration requirements as outlined in or derived from the
NIRSpec FRD. The NIRSpec Calibration Plan document will consist of several volumes
including: 1) the routine calibrations planned during the use of NIRSpec in flight, 2) the ground
test plan (NIRS-CRAL- PL-0001 issue 1 or its latest revision), and 3) the in-orbit checkout or
verification plan.
1.2
Various Goals of Calibration
The motivations for calibration activities evolve through the various stages in the life of an
instrument, but fall into one of the following categories:
1. Characterizing the properties or behavior of a component of the instrument or the
integrated instrument, in order to verify that baseline specifications are met.
2. Optimizing the adjustable parameters associated with one or more component of the
instrument to ensure the required performance is achieved.
3. Monitoring the performance of the instrument in order to document and mitigate any
degradation that might occur over time or with different environmental circumstances.
4. Preparing the data returned from the instrument for scientific analysis and interpretation
by removing instrumental signatures and artifacts.
Characterization and optimization are emphasized during assembly, integration, and test (AIT) of
the instrument. Many of these aspects of understanding the behavior of the instrument are
revisited during the on-orbit commissioning phase as well, when the actual operational
conditions have been attained. Thereafter, routine monitoring of on-orbit performance, with
occasional changes in adjustable parameters to optimize performance in the face of aging and
other effects is required.
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The fourth goal is qualitatively different, since it involves manipulations to the data returned by
the instrument rather than measuring the performance of the instrument itself. Of course,
without detailed understanding of the behavior of the instrument and its key components, the
manipulations required to remove instrumental signatures cannot be performed with the desired
level of fidelity. The information necessary to accomplish these tasks is usually communicated
via a variety of reference files, which are the primary by-product of the activities that are directly
linked to characterizing the hardware.
1.3
NIRSpec Instrument Purpose
The purpose of the Near-Infrared Spectrograph (NIRSpec) is to provide low,
medium, and high-resolution spectroscopic observations over the wavelength
range 0.6 µm – 5 µm in support of the four JWST science programs:
1.
2.
3.
4.
Identification of the “first light”
Structure formation in the universe and the assembly of the first galaxies
Formation of stars and planetary systems
Evolution of planetary systems
A complete description and discussion of the NIRSpec science requirements is listed in the
James Webb Space Telescope Project Science Requirements Document (JWST-RQMT-002258).
These requirements flow to and are expanded into additional specific requirements for the
instrument in the NIRSpec Functional Requirements.
1.4
NIRSpec Instrument Design
NIRSpec is a multi-object dispersive spectrograph covering a field-of-view (FOV) of > 3 x 3
arcmin, capable of observing >100 sources simultaneously in a variety of passbands from 0.6 to
5. Microns. The European Space Agency (ESA) is providing the instrument for the James Webb
Space Telescope (JWST). EADS Astrium and various subcontractors are building NIRSpec for
ESA. SI PI Dr. Peter Jakobsen (ESTEC) leads the IDT.
The region of sky to be observed is transferred from the JWST optical telescope element (OTE)
to the spectrograph aperture focal plane (AFP) by a pick-off mirror (POM) and a system of foreoptics which includes a filter wheel for selecting band passes and introducing internal calibration
sources. The nominal scale at the AFP is ~2.5 arcsec/mm.
For R=100, 1000, or 2700 spectroscopy, targets in the FOV are normally selected by opening
groups of shutters in a micro-shutter array (MSA) in specified patterns to form slits. In
addition to the apertures defined by the MSA, there are five fixed-slits in the AFP that can be
used for high-contrast R=100 or R=1000 spectroscopy. There also is an integral-field unit (IFU)
that uses a fixed entrance slit with a 3x3 arc sec FOV for R=2700 integral-field spectroscopy
with average spatial sampling of 0.075 arcsec.
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These slits are re-imaged onto a two-element mosaic of NIR detectors (the focal-plane array:
FPA) by a collimator, a dispersing element (gratings or a double-pass prism) or an imaging
mirror, and a camera. The image scale on the FPA is nominally 5.56 arcsec/mm (100 mas per
detector pixel).
In separate pairs of columns Table 1 lists each optical element and its corresponding resolution,
the combinations of wavelength region and filter, and the slits and associated fields-of-view for
the instrument. No combinations of slits and spectral elements are precluded from use, however
Table 2 lists the various combinations of these quantities that are anticipated to be most
commonly used and are presently considered the standard observing modes. The basic elements
of the spectrograph are illustrated schematically in Figure 1, which shows both the optical
subsystems and associated mechanisms. Figure 2 shows the slit mask overlaid on the detector
assembly.
Optical
Element
Spectral
Resolution
Wavelength
range (µm)
Filter Wheel
Slit
FOV
Mirror
---
0.6-5.0
Transparent
MSA
3.4x3.5 arcmin
P285L
100
>1.0
Long pass I
IFU
3x3 arcsec
G140M
1000
>1.7
Long pass II
SLIT_A_200_1
200x3500 mas
G235M
1000
>2.9
Long pass III
SLIT_A_200_2
200x3500 mas
G395M
1000
0.96-1.24
Broadband A
SLIT_A_400
400x4000 mas
G140H
2700
0.78-2.10
Broadband B
SLIT_A_100
100x2000 mas
G235H
2700
0.7-5.0
Long pass 0.7
SLIT_B_200
200x3500 mas
G395H
2700
MSA: 4x365x171 shutters; each shutter 200 mas x 450 mas with 264x514 mas spacing; FOV: 9
arcmin2 (3.4x3.5 arcmin)
IFU: one slit; average spatial sampling 0.075 arcsec; FOV: 9 arcsec2 (3 x 3 arcsec)
Fixed slits: 5 slits
Pairs of columns list optical element and corresponding resolution, the combinations of wavelength region and filter,
and slits and their fields-of-view. All combinations of slit and spectral element are allowed.
Table 1: NIRSpec Optical Element, Bandpass, and Slit characteristics (all modes).
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Figure 1: Schematic diagram showing the layout of the NIRSpec.
Figure 2: Schematic layout of the NIRSpec slit mask overlaid on the detector array
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Mode
Imaging
Imaging
Imaging
Imaging
Imaging
Imaging
Imaging
Spectroscopy
Spectroscopy
Spectroscopy
Wavelength Filter Wheel
range (µm)
0.6-5.0
>1.0
>1.7
>2.9
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AFP
Resolution
Optical
Element
0.7-5.0
transparent
Long pass I
Long pass II
Long pass III
Broadband A
Broadband B
Long pass 0.7
TA config
TA config
TA config
TA config
TA config
TA config
TA config
0.6-5.0
transparent
MSA
R=100
1.0-5.0
0.6-5.0
1.0-5.0
Long pass I
transparent
Long pass I
MSA
Fixed slit
Fixed slit
R=100
R=100
R=100
P285L (Dualpass prism)
P285L
P285L
P285L
0.7-1.4
Long pass 0.7
R=1000
G140M
1.0-1.8
Long pass I
R=1000
G140M
1.7-3.0
Long pass II
R=1000
G235M
2.9-5.0
Long pass III
MSA,
Fixed slit
MSA,
Fixed slit
MSA,
Fixed slit
MSA,
Fixed slit
R=1000
G395M
0.7-1.4
Long pass 0.7
R=2700
G140H
1.0-1.8
Long pass I
R=2700
G140H
1.7-3.0
Long pass II
R=2700
G235H
2.9-5.0
Long pass III
MSA, Fixed
slit, or IFU
MSA, Fixed
slit, or IFU
MSA, Fixed
slit, or IFU
MSA, Fixed
slit, or IFU
R=2700
G395H
0.96-1.24
0.78-2.10
Mirror
Mirror
Mirror
Mirror
Mirror
Mirror
Mirror
Table 2: NIRSpec SI characteristics (standard modes)
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Supporting Documents and Presentations
Document
Author(s)
JWST–RQMT–002060 Draft
JWST-RQMT-000835
JWST-RQMT-002558
JWST-RQMT-000634
JWST-RQMT-02020
STScI-JWST-OPS-002
ST-ECF Instrument Science Report
JWST 2003-01
ST-ECF Instrument Science Report
JWST 2004-01
CRAL-PJT-NIRS-CAL-PLN-20050201
STScI-JWST-R-2003-0003
STScI–JWST–TM–2004–0025
STScI–NGST–R–0014A
NIRSpec Definition Study – Final
Presentations; 11 May 2004
NIRSpec IDT Calibration Meeting
Presentations; 16/17 June 2005
Kuntschner et al
Kuntschner et al
Ferruit
Regan et al
Kriss
Casertano
P. Ferruit, others
P. Jakobsen, T.
Boeker, P. Ferruit,
others
ESA-JWST-RQ-322
Title
Near-Infrared Spectrograph Functional Requirements
Document (FRD), (draft 15 Feb 2005)
JWST ISIM Requirements Document
JWST Project Science Requirements Document (SRD)
JWST Project Mission Requirements Document (MRD)
JWST Observatory Requirements Document
Operations Concept for the JWST Mission
Calibration Concept for the JWST Near-Infrared
Spectrograph (NIRSpec), ver 1.2, 18 June 2003
Ground Calibration Concept for the JWST NearInfrared Spectrograph (NIRSpec)
JWST/NIRSpec On-ground Calibration Plan
NIRSpec Operations Concept, issue 2.0, 10 Nov 2003
Recommendations for JWST FITS Formats and
Keywords
NGST Calibration Overview
Various presentations
Various presentations
ESA NIRSpec System Requirements Document (ESA
SRD), issue 2, 10 Feb 2005
Near Infrared Spectrograph for JWST – Design
Development and Verification Plan
Near Infrared Spectrograph for JWST – Assembly,
Integration, and Test Plan, Issue 2, 4 Oct 2004
JWST NIRSpec System Requirement Document, Issue
1, rev 2 draft, 18 Nov 2004
NIRSpec detector sub-system functional and
performance requirements specification, issue 2, rev
1(draft) 25 June 2004
NIRSpec micro-shutter array sub-system functional and
performance requirements specification, issue12, rev 0
12 May 2003
NIRS-ASD-PL-005
NIRS-ASD-PL-0012
ESA –JWST-RQ-322
ESA –JWST-RQ-22
ESA –JWST-RQ-22
NIRSpec IDT draft memo, 15 June
2005
CRAL-PJT-NIRS-CAL-TN-20050101, draft 1 Mar 2005
CRAL-PJT-NIRS-CAL-TN-20011101, draft 3 Nov 2004
CRAL-PJT-NIRS-CAL-TN-20041002, draft rev 17 Nov 2004
NIRS-CRAL-PL-0001, issue 1 23
Nov 2005
De Marchi, G. and
Boeker, T.
Ferruit, P.
NIRS-CRAL-RQ-0001
Ferruit, P.
Ferruit, P.
Ferruit, P.
Ferruit, P.
NIRSpec pipeline concept – a high level description
Review of the in-orbit radiometric calibration of the
NIRSpec instrument
Review of the calibration of distortion for the NIRSpec
instrument
Review of the in-orbit wavelength calibration of the
NIRSpec instrument
JWST NIRSpec Performance & Calibration on-ground
calibration plan for the NIRSpec FM-level calibration
campaign
NIRSpec OGSE Requirements
Table 3: NIRSpec-related Documents
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Sources of Calibration Information
Quantitative information about various aspects of the performance of the NIRSpec will be
available from a variety of sources, which change through the lifetime of the instrument. In
general terms, these can be characterized as follows:
1. Component: Ground-based testing of individual components during assembly. For
example, the reflectivity characteristics of a grating or the band-pass of a specific filter
can be most easily characterized before it is installed in the instrument.
2. I & T, Thermal-Vac: Ground-based testing of the integrated or partially integrated
instrument during I&T, optical end-to-end tests, or space environment testing. Such tests
provide initial information concerning, e.g., the overall sensitivity of the instrument. This
phase of testing will provide valuable data that will be used in production of a variety of
initial calibration reference files.
3. On-orbit dedicated: On-orbit observations dedicated to NIRSpec calibration activities,
either of the built-in calibration lamps or specially selected astronomical sources. These
observations will be obtained in both the commissioning period and during the routine
science operations lifetime.
4. On-orbit parallel: On-orbit observations done in parallel with science or calibration
observations conducted by different primary instruments. Such parallel observations will
likely be confined to the built-in calibration lamps or dark exposures. These observations
will be obtained in both the commissioning period and during the routine science
operations lifetime.
5. On-orbit as science: Primary science data collected by the NIRSpec on-orbit. Such data
serve as the primary resource for the detection of subtle, systematic deficiencies or
changes in the overall quality of the science data.
6. Simulations and modeling: Instrument simulations and calibration models will be
useful guides and aids in certain calibrations. (dispersion from optical design and
principles, flat fields based on ground-based observation, instrument simulations like
those provided by the NIRSpec instrument performance simulator (IPS).)
2.1
Overview of Assembly, Integration and Testing
The IDT has prepared a comprehensive, preliminary ground calibration plan for the FM (NIRSCRAL-PL-0001 dated 23 November 2005, P. Ferruit). The ground-based calibration campaigns
will be aimed at characterizing and calibrating the instrument and, in certain cases, may be used
for verification purposes. Many of the activities will be conducted at operating (cryogenic)
temperatures. Ground-based NIRSpec calibrations will be conducted at several stages of
instrument construction with a variety of Ground Support Equipment. Some individual
components must be tested separately, subsystem tests will be conducted on a development
model (DM) and Engineering Test Unit (ETU) of the SI, and full system-level testing will be
conducted with the Flight Model (FM) of the instrument. Early cryogenic testing will be
performed on the DM. A refurbished DM will become the ETU, which will be delivered to
NASA/GSFC for incorporation in the ISIM ETU Integration & Test program, and will validate
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the procedures to be used in the Flight ISIM I&T program. Early component or subsystem
testing can occur individually or at the DM/ETU level. Prior to assembly, the transmitted or
reflected figures of all optical components will be verified, as will their optical properties in the
spectral region of interest. Components will also be rigorously inspected for damage. At present
no specific detailed component or subsystem calibration campaigns have been fully defined
either by the IDT or at GSFC.
2.1.1
DM/ETU
The DM/ETU is planned to be optically representative of the flight instrument up to the plane of
the MSA with representative fore-optics and a non-functional, single-position filter wheel and an
ETU FPA in the MSA plane. This system will be essentially an imaging device that will be
useful for early characterization and calibration of the spatial aspects of the SI, probably limited
PSF and distortion measurements. There will be no functional internal calibration channel in the
DM/ETU.
2.1.2
FM
The FM will be the actual fully integrated flight instrument with full design capabilities. It will
have an active internal calibration channel. Ground testing of the FM will provide most of the
pre-flight knowledge of the SI characteristics. All modes and instrument configurations will be
tested to provide a complete set of instrument-level calibrations, validation of the TA algorithms,
and refinement and verification of the important, planned on-orbit calibration procedures. Some
of the ground calibrations of the FM will be used for SI verification purposes.
At present the project plans no ISIM- or observatory-level calibration activities apart from basic
alignment and performance checking.
2.1.3
Optical GSE
A detailed description of the optical ground support equipment (OGSE) that will be used during
the verification and calibration campaign (in particular for the cryogenic testing of the FM) can
be found in Sect. 9.1 of the NIRSpec Assembly, Integration and Test Program (NIRS-ASD-PL012). In this document, we include the key parameters of the OGSE.
OGSE is a large set of equipment that not only is necessary to provide the signal input into the SI
in system-level environmental and cryogenic testing, but also will be used for integration and
testing in ambient conditions as well. Based upon detailed ground calibration needs, the
NIRSpec IDT is presently considering the precise requirements and specifications for the OGSE
(see NIRS-CRAL-RQ-0001 presently in draft).
The OGSE will be a complex assortment of hardware that will be essentially a telescope
simulator. It must be operated at cryogenic temperatures and will include a number of
mechanisms with stringent positioning and stability requirements. Several types of illumination
will be required with an integrating sphere in OGSE: flat field illumination with typical field
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stop masks, a grid of spatially resolved sources for distortion measurements, a grid of nominal
point sources for PSF measurements, TA checking, and relative photometric calibration, and by
itself, a single, precisely radiometrically calibrated source for the absolute photometric
calibrations. A variety of input sources will be needed; e.g., narrowband and emission line
sources, and broadband continuum sources – the exact properties of these sources is presently
being considered by the IDT. The positioning of input source illumination masks should be
controlled very precisely in order to allow micro positioning of sources within instrument slits.
The sources themselves should be movable on larger scales in order to move accurately within
much of the SI FOV.
2.1.4
Electrical GSE (EGSE)
Communications with the NIRSpec instrument occurs via the instrument command & datahandling computer (ICDH) . This interface will be used for most instrument-level testing via
dedicated EGSE. In particular, the Science Instrument Test Set (SITS) supplied by the JWST
Project at NASA/GSFC will be used to operate the NIRSpec during verification testing. The
SITS provides a simulation of the ISIM system elements from the NIRSpec interfaces to the
ground system, including ground command inputs and science data and engineering
housekeeping data outputs.
The SITS consists of two major components: a Science Instrument Development Unit (SIDU)
and a Space Wire Test Set (SWTS). The SIDU will be used to:
•
•
•
•
develop NIRSpec flight software to control and monitor the instrument mechanisms.
perform initial development and test of the instrument control electronics (ICE)
hardware.
develop, verify, and validate instrument test procedures for use during instrument- and
ISIM-level I&T.
develop, verify, and validate command and telemetry database definitions that will be
used by the JWST flight software database.
The SWTS will be used for initial development and test of the Space Wire interfaces between the
ISIM and the NIRSpec ICE/FPE (focal plane electronics) units.
The NIRSpec Instrument Data Handling and Analysis System (DHAS) is the final component of
the EGSE. The DHAS will be a basic workstation with customized software to enable offline
analysis of NIRSpec image data. The DHAS will extend the ESA/NASA provided SITS in order
to verify the performances and to calibrate the Instrument.
The DHAS will ingest FITS file formatted science raw data from the SITS and archive all these
data with related time and/or observation ID. From the data storage the data handling and
analysis system users will have the possibility via the attached stations to select and access the
data for nearly real-time, retrieve or playback processing. A hierarchical file management system
functionality support easy searching, browsing and recovery of archived data based on time
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and/or observation ID. This data selection and access possibility will be as well provided to the
users of the ESA science station and to the GSFC provided station.
The data handling and analysis system will incorporate the necessary hardware and software for
the instrument verification and calibration.
One part of the system will be used for scientific calibration as well as for instrument
verification. This will as well include the system to ‘quick look’ display and analyse the science
data (IQLAC).
The Quick Look Analysis System provided (HW & SW) by GSFC to verify the operation and
health and safety of the FPA will be required to archive its own products.
The following physical stations will build up the DHAS:
• Server for Storage, data management, data switch & FTP/internet server (SSDS)
• Instrument Verification, QLA and Calibration HW/SW provided by LAM (IQLAC)
• FPE/FPA Quick Look Analysis HW/SW provided by GSFC JWST (FPQLA)
Additional to the stations described above the data handling and analysis system provides the
capability to connect and interface to
• The ESA Science Calibration Station(s)
• The Instrument Performance Simulator (IPS)
Thus, the NIRSpec DHAS will be the focus of efforts to characterize the performance of the
NIRSpec and generate an initial suite of reference files for the calibration pipeline (see Section
5.5).
2.1.5
Ambient Ground Calibration
Some components, such as mirrors, filters, and detectors may be tested at room temperature as
well as operating temperature at ambient pressure. Some alignment activities can also be
performed at ambient conditions. Wave-front error maps and distortion maps will be
characterized in ambient conditions. Indeed the OTE-focal-sphere to MSA distortion map will
be characterized at ambient and will not be directly measurable after this phase. On the other
hand, certain important components may not be fully operable at ambient (e.g., the FPA and
MSA).
2.1.6
Thermal-Vac
The bulk of the ground-based data that will serve as input into the generation of calibration
reference files will be obtained with the FM in temperature and pressure conditions typical of
actual flight conditions at L2. This will require that the instrument be cooled to ~37° K in a
suitable Thermal-vacuum (TV) chamber in which it can receive the necessary input signals from
the OGSE. At this time it is envisioned that 30 days of 24/7 cryogenic, vacuum testing will be
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available. The verification of the flight unit will include a combination of functional tests during
integration and subsequent environmental simulations under TV conditions.
NSFR-17 Prior to launch, the NIRSpec shall survive at least 4 cryocycles from
ambient to survival temperature during ISIM and Observatory level testing.
2.2
Overview of On-Orbit Commissioning
A detailed plan for the functional tests and calibration observations during on-orbit
commissioning of the NIRSpec has not been developed yet. Section 4.1.1 contains a listing of
possible commissioning activities. The duration and depth of commissioning functional tests
and calibration activities will ultimately be dictated by NIRSpec FRD requirement NSFR-20,
which states that NIRSpec shall complete its commissioning activities within 90 days (TBR) of
reaching operating temperature. NFSR-16 also implies that the maximum duration of the
commissioning period would be 6 months. SI BOL commences at the end of the commissioning
period and nominal EOL follows 5 years later. Full SI performance requirements are to be met
at EOL and some include reference to BOL, as well.
NSFR-16 All NIRSpec components and sub-systems shall survive 5.5 years of nominal in-orbit
operations (incl. 6 months for commissioning), plus 3.5 years of testing prior to launch, plus 3
years of ground storage.
NSFR-20 NIRSpec shall complete its commissioning activities within 90 days (TBR 1) of
reaching operating temperature using no more than 25% (TBR 2) of the available observatory
time.
NSFR-2 All requirements in this section shall be fulfilled at EOL except when mentioned
otherwise.
Before genuine calibration activities can begin, the NIRSpec will have to undergo a suite of
housekeeping, basic aliveness, and functional tests in order to establish, e.g., whether the grating
and filter wheels and other mechanisms respond correctly to commands; whether the calibration
sources work; and so on. We will not enumerate all of those here. However, in section 4 we
will mention those that are directly related to or facilitate calibration-type activities. Some issues
associated with this phase of in-orbit checkout include:
• How cool does the ISIM cavity have to be before these tests can begin? It is possible that
the ISIM will have to be quite near its nominal operating temperature (~37° K) before
functional tests involving the MSA can begin, due to bowing at higher temperatures. The
temperature and pressure at which it will be possible to operate the MSA shutters is TBD
at present.
• The present baseline is for this testing to be performed sequentially in the commissioning
period. Can some tests of NIRSpec be run at the same time as (parallel operation with)
commissioning tests of other SIs or other Observatory activities, or must they be run
sequentially in the commissioning period?
Meaningful external scientific calibration of the NIRSpec cannot begin before the nominal
operating temperature is reached and the wave-front sensing and control (WFS&C) process has
converged to provide a stable point-spread function.
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Onboard Calibration Lamps
The specification for the onboard calibration unit is presently being put together by the IDT. The
practicalities of the instrument layout led to a design with a mirror located on the back of the
filter wheel shutter to inject light into the optical path. As a result light from the onboard
calibration unit does NOT pass through the fore-optics or the filter wheel, hence does not follow
the same exact path as light from external sources. There will be a set of continuum and spectral
sources to facilitate flat field and wavelength calibration of the SI, although the exact set of
sources is still under evaluation. The unit will provide a flat field type of illumination of the
FOV using an integrating sphere. Presently filaments with no glass enclosure are being
considered for the continuum sources. The spectral line sources are not yet decided. Two
different onboard calibration unit configurations are under consideration in order to provide the
maximum unique flat field and wavelength calibration coverage for the instrumental modes.
Calibration Type
Lamp
Flat Field
5 continuum lamps
Wavelength
4 spectral lamps
Table 4: Calibration Sources in NIRSpec
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Assessment of Ground Calibration Needs
In this section, the calibrations required to characterize the behavior of the NIRSpec are
discussed on a component-by-component and calibration-type basis. Particular attention is paid
to the needs of ground calibration, however the need for on-orbit calibration is indicated as
necessary. The approach is to summarize the function and influence of the various optical
components. Specific items requiring characterization are identified, as are the signatures that
are imprinted on image data. Mitigation strategies are discussed briefly. Following the
component-by-component and assembly listing, specific types of end-to-end calibrations
required at system or integrated SI level are described. Input from the ECF calibration concept,
ground-calibration concept, and ground calibration plan (NIRS-CRAL-PL-0001, P. Ferruit)
documents are included throughout this section.
The next several sections provide a component-by-component and assembly listing in which a
template format is used to describe each component, list properties to be verified, indicate the
influence on SI performance and data, include any remarks pertaining to calibration of the
component, and to list anticipated testing environments for the component. After the
component-by-component narratives, there are sections for the subsystem, DM/ETU, and FMlevels of characterization on a calibration type-by-type basis.
3.1
Transmissive Optical Components (6)
There are six filters that will be utilized in NIRSpec. Four long-pass filters will be used in
conjunction with the gratings and the imaging mode to isolate the specific spectral regions to be
observed. Two special-purpose filters (Band “A” and Band “B”) are also proposed for imaging
mode. Additionally, there will be several transmissive components in the calibration assembly.
3.1.1
•
•
•
•
•
•
Component Properties to be verified and characterized
Measure transmission as function of wavelength; meets specifications
Measure surface flatness, figure, and wedge
Evaluate surface roughness and defects
Measure bandpasses for each filter
Suppression of unwanted reflections from filters
Transmissive characteristics of calibration assembly components
3.1.2 Influence on NIRSpec performance and data
The transmission of the filters contributes to the sensitivity of the NIRSpec, but sensitivity is best
characterized as an instrument-level throughput. Roughness or other filter surface quality factors
may degrade the final point-spread function (PSF) or contribute to scattered light.
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3.1.3 Remarks
Characterization and verification of the properties of the various filters are most easily
accomplished before they are installed in the NIRSpec. The bandpasses and profiles of the
blocking filters will be most easily measured during ground-based AIT activities. Thereafter,
they can be monitored for degradation (as a function of age or temperature) via their contribution
to the total throughput of the instrument. It will be more difficult to monitor shifts in the
bandpass as a function of age. ECF ground-calibration concept recommends characterization at
ambient and at mean ISIM temperature.
Anticipated testing environment: Component
3.2
Dispersive Elements Gratings (7)
Six gratings (G140M, G235M, G395M, G140H, G235H, and G395H) and one prism (P285L)
are utilized as dispersive elements in NIRSpec. The satisfaction of the following two FRD
requirements are directly related to their characteristics:
NSFR-39 NIRSpec shall obtain low-resolution (R=100) spectra over the wavelength range 0.6
µm - 5.0 µm in a single exposure.
NSFR-40 NIRSpec shall obtain medium (R=1000) and high (R=3000) resolution spectra over
the wavelength range 1.0 µm - 5.0 µm in no more than three separate exposures.
3.2.1
•
•
•
•
•
•
Component Properties to be verified and characterized
Efficiency as function of wavelength; efficiency meets specifications
Resolution characteristics for zeroth, first, and second orders
Scattered light parallel to and perpendicular to dispersion
Ghosts in dispersed image
Polarization introduced by dispersive elements
Optical surface meets specifications
3.2.2 Influence on NIRSpec performance and data
The efficiency of the various NIRSpec dispersive elements contributes to the sensitivity of the
NIRSpec, but sensitivity is best characterized as an instrument-level throughput. Roughness in
the optical surface of the gratings may degrade the final line-spread function (LSF), hence the
spectral resolution and scattered light characteristics.
3.2.3 Remarks
Characterization and verification of the properties of the various dispersive elements are most
easily accomplished before they are installed in the NIRSpec. Tests are likely to be performed as
part of the flight dispersive optics selection process. Both reflective and transmissive
characteristics of the prism must be characterized. ECF ground-calibration concept recommends
characterization at ambient and at mean ISIM temperature.
Anticipated testing environment: Component
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Non-dispersive Reflective Optical Elements (Mirrors) (14)
There are 14 reflective surfaces in the NIRSpec optical path including the collimator, camera
assembly, the pick-off mirror (POM) flat, and fore optics, the focusing assembly, 8 additional
reflective surfaces in the IFU, and additional elements in the calibration system optical path.
3.3.1
•
•
•
•
Component Properties to be verified and characterized
Reflectivity as function of wavelength; reflectivity meets specifications
Collimation meets tolerance
Optical surface meets specifications
Fore optics wave-front errors
3.3.2 Influence on NIRSpec performance and data
The reflectivity of the POM, the collimator, and other various NIRSpec reflective optical
elements contribute to the sensitivity of the NIRSpec, but sensitivity is best characterized as an
instrument-level throughput. Roughness in the optical surfaces of the mirrors may degrade the
final point-spread function (PSF).
3.3.3 Remarks
Characterization and verification of the properties of the various mirrors are most easily
accomplished before they are installed in the NIRSpec. ECF ground-calibration concept
recommends characterization at ambient and at mean ISIM temperature.
Anticipated testing environment: Component
3.4
Opto-mechanical elements (grating wheel, e-focus mechanism, etc) (3)
Several mechanical elements present in the optical path can affect observational quality and
hence are of interest in calibration considerations. These include the filter wheel, the grating
wheel, and focusing mechanisms. The following FRD requirement directly pertains:
NSFR-61 Once in position, the NIRSpec mechanisms - grating wheel, filter wheel, and
refocusing mechanism(s) - shall not produce any micro-vibrations.
3.4.1
•
•
Component Properties to be verified, defined, characterized, or optimized
Positional repeatability
Mechanism stability
3.4.2 Influence on NIRSpec performance and data
The stability of these elements would contribute to the image quality, the spectral resolution and
range, and the quality of the wavelength calibration of NIRSpec, but these quantities are best
characterized at the instrument-level.
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3.4.3 Remarks
Characterization and verification of the repeatability and stability properties of these mechanisms
can be demonstrated at the component level, however the implications of NSFR-61 suggest that
these properties should be evaluated at all stages of integration and operation from component
level though flight operations. Characterization should occur at ambient and at mean ISIM
temperature.
Anticipated testing environment: Component, T-V level, ISIM, on-orbit
3.5
Camera Systems (3 elements)
The point-spread function from the cameras is under sampled at the detector for all wavelengths
short ward of approximately 3 microns.
3.5.1
•
•
•
Properties to be defined and characterized
Focus (particularly at cryogenic operating temperatures)
The point-spread function (PSF) as a function of position
The geometric distortion in the image plane
3.5.2 Influence on NIRSpec performance and data
Specific imprints on the NIRSpec images attributable to the cameras are
• The final PSF
• Geometric distortion
3.5.3 Remarks
During routine, on-orbit operations, the under-sampling of the PSF, particularly in the spatial
direction, over most of the NIRSpec small-scale dithering will mitigate waveband. Dither
schemes should be tested in ground based testing at the DM/ETU and FM level. The geometric
distortion of the entire optical system can be characterized during AIT by detailed ray tracing
and by imaging a mask containing regular grid of holes. Determinations on-orbit will be a byproduct of the astrometric calibration (Section 4.2). Characterizations should occur at mean
ISIM temperature.
Anticipated testing environment: Unit, T-V, on-orbit
3.6
Apertures (fixed and MSA) (5 fixed slits; IFU aperture; MSA)
Several fixed slits are available for use with all dispersers. An additional fixed aperture is
dedicated for use with the IFU. The following FRD requirements concern the physical
characteristics of the apertures:
NSFR-41 In R=100 mode, the spectral resolution over the spectral range 1.0 µm – 5.0 µm shall
not exceed the range R = 50 - 200 for a 200 mas wide slit or MSA shutter.
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NSFR-54 The NIRSpec optics shall be sufficiently oversized so that no more than 10% of the
light passing through the slit is lost due to diffraction effects. This applies to all wavelengths
between 0.6 µm and 5.0 µm and all positions within the NIRSpec FOV.
NSFR-8 NIRSpec shall have a dedicated aperture for high-contrast, single-object spectroscopy
with an out-of-slit rejection ratio higher than 106, i.e. less than 1 part in 106 shall be transmitted
outside of the slit.
3.6.1
•
•
•
•
•
Component Properties to be verified and characterized
Physical size of each aperture
Location of each aperture within the Aperture Focal Plane (AFP)
Location of corners or other readily identifiable physical features
Out-of-slit rejection
Characterize aperture diffraction
3.6.2 Influence on NIRSpec performance and data
The sizes of the various NIRSpec apertures can contribute to the sensitivity of the NIRSpec, but
sensitivity is best characterized as an instrument-level throughput.
3.6.3 Remarks
Characterization and verification of the properties of the various apertures are most easily
accomplished before they are installed in the NIRSpec. ECF ground-calibration concept
recommends characterization need be performed only at ambient temperature.
Anticipated testing environment: Component
3.7
Integral Field Unit
Since the operation of the IFU is substantially different in concept from the other data-taking
slits, there will be a separate section later in this chapter (section 3.19) dealing specifically with
the spectroscopy-related calibration characteristics of the IFU unit. However, we list here
quantities that pertain to the component-level characteristics of the IFU. The NIRSpec Integral
Field Unit will sample a small subset of the overall FOV and slice it to provide an R=2700 data
cube over the entire slit area. The following FRD requirements pertain to or derive from IFU
characteristics:
NSFR-62 NIRSpec shall incorporate an IFU to enable high spectral resolution (R=3000)
observations of a contiguous area of the sky no smaller than 3’’ x 3’’ over the 1.0 µm - 5.0 µm
spectral range in no more than three separate exposures.
NSFR-63 NIRSpec shall provide target acquisition functions, which shall enable autonomous
positioning of the IFU FOV over the selected target region.
NSFR-64 In IFU mode, the average optical throughput of the NIRSpec optical train shall be ≥
20% including the relevant R=3000 dispersive element and the relevant order blocking filter for
any wavelength between 1 µm and 5 µm and for any position within the IFU FOV.
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NSFR-65 The IFU shall have the same axial focus position with the other NIRSpec operational
modes.
3.7.1
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•
•
Component Properties to be verified and characterized
Measure reflectivity for all optical elements in IFU
Measure scattered light characteristics for all optical elements in IFU
Optical surfaces meet specifications; alignments at detector
3.7.2 Influence on NIRSpec performance and data
The efficiency of the various IFU elements contributes to the sensitivity of the NIRSpec, but
sensitivity of the IFU is best characterized as a system-level throughput for the IFU unit.
3.7.3 Remarks
Characterization and verification of the properties of the various IFU elements are most easily
accomplished before they are installed in the NIRSpec. Subsequent system level
characterizations of the integrated IFU unit should be performed to assess overall throughput and
image quality. ECF ground-calibration concept recommends characterization at ambient and at
mean ISIM temperature.
Anticipated testing environment: Component, system (unit) level.
3.8
Calibration Assembly
The NIRSpec calibration unit assembly is presently being defined. The unit will provide flat
field and wavelength calibration capabilities. The following FRD requirements pertain to
calibration unit characteristics:
NSFR-66 NIRSpec shall carry an internal light source with a spectrum free of any prominent
emission lines or absorption features (“continuum source”) in order to enable calibration of the
detector response pattern (“flat field”).
NSFR-67 NIRSpec shall carry an internal light source with a sufficient number of narrow, welldefined spectral features (“line source”) in order to enable calibration of the wavelength scale in
NIRSpec spectra.
NSFR-68 All NIRSpec-internal calibration sources shall illuminate the entire FOV with a spatial
uniformity of 25% or better.
NSFR-69 All NIRSpec-internal calibration sources shall illuminate any 5-arcsec-diameter region
of the FOV with a spatial uniformity of 1% or better.
NSFR-70 The flux of all NIRSpec-internal calibration sources shall degrade less than 10%
between BOL and EOL.
3.8.1
•
•
•
Component Properties to be verified and characterized
Illumination uniformity of lamps
Applicability of internal lamps
S/N versus exposure time
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Identify lamp heating, aging, and stability characteristics
Evaluate safety and image quality issues for SCA (e.g., persistence)
Detailed “line” maps for wavelength calibration lamps
Reflectivities of whole assembly as function of wavelength
3.8.2 Influence on NIRSpec performance and data
The calibration unit components affect accurate determination of observational image quality,
spectral resolution, and wavelengths.
3.8.3 Remarks
All properties listed above for characterization should be determined as a function of lamp
operating current. Verification of the properties of the lamps is most easily accomplished before
they are installed in the NIRSpec. Subsequent system level characterizations of the integrated
calibration unit should be performed to assess flat fielding and wavelength calibration capability.
Characterization of all characteristics of the integrated unit should be performed at nominal ISIM
temperature and possibly bracketing values.
Anticipated testing environment: system (unit) level.
3.9
Micro-Shutter Array (MSA)
The MSA unit allows multi-object spectroscopy with NIRSpec. The following FRD requirement
pertains to MSA unit characteristics:
NSFR-41 In R=100 mode, the spectral resolution over the spectral range 1.0 µm – 5.0 µm shall
not exceed the range R = 50 - 200 for a 200 mas wide slit or MSA shutter.
NSFR-58 The flux transmitted through an MSA shutter in the “closed” state shall be less than
1/2000 of the flux transmitted through a shutter in the “open” state.
3.9.1
•
•
•
•
•
Component Properties to be verified and characterized
Defective shutter map
Contrast ratio between open and closed shutters, light tightness of assembly
MSA-introduced polarization
Wave-front errors as function of position in MSA
Characterization of the MSA BRDF and BTDF
3.9.2 Influence on NIRSpec performance and data
The efficiency of the MSA aperture contributes to the sensitivity of the NIRSpec, but sensitivity
is best characterized as an instrument-level throughput. Throughput variations introduced by
diffraction on the MSA support structure and target misplacement relative to the fixed MSA grid
can be reduced or removed by appropriate dithering patterns. The optimal dithering patterns
need to be evaluated with accurate modeling.
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3.9.3 Remarks
Characterization and verification many of the properties of the MSA are most easily
accomplished before unit installation in the NIRSpec. Dither schemes should be tested in ground
based testing at the DM/ETU and FM level. ECF ground-calibration concept indicates defective
shutter map and polarization measurements may be performed at ambient with the balance of the
tests conducted at nominal MSA temperature, however due to present concerns, the feasibility of
operating the MSA at ambient temperatures must be established.
Anticipated testing environment: Component, system (unit) level
3.10 Sensor Chip Assemblies (detectors) (2)
The NIRSpec has two Sensor Chip Assemblies (SCAs) to record the arrival and location of
photons. Each will be a 2048 x 2048 HgCdTe array with 18-micron pixels, good quantum
efficiency, and low read noise. The baseline plate scale is 0.1 mas/pixel. The NIRSpec does
place more stringent requirements on its detectors than NIRCam, but many of the calibration
activities are likely to parallel those of other JWST science instruments (SIs), particularly
NIRCam. However, since most NIRSpec usage will be for faint source spectroscopy, the
limiting NIRSpec signal-to-noise ratio (SNR) will generally be determined by the read-noise of
the SCAs.
The following FRD requirements pertain to SCA characteristics:
NSFR-49 The mean detective quantum efficiency (DQE) of the NIRSpec SCAs (averaged over
all operable pixels) shall be higher than 80% for all wavelengths between 1 µm and 5µm, and
higher than 70% for all wavelengths between 0.6 µm and 1µm.
NSFR-50 The pixel-to-pixel variation of the DQE shall be less than 10% root mean square
(RMS) when measured over all operable pixels.
NSFR-51 The NIRSpec detector system including FPA, FPE and harness shall have a mean total
noise per pixel of less than 6 e-, where the mean is computed over all operable pixels.
Note: “Total noise” is the sum of all contributions that affect the noise level, such as the read-out
electronics, on-chip circuitry, dark current, cabling, etc. It shall be computed from 1000s long
MULTIACCUM84 exposures.
NSFR-52 The NIRSpec SCAs shall have a well capacity of at least 60,000 e- per pixel, averaged
over all operable pixels.
3.10.1 Properties to be defined, characterized, or optimized
In principle, the following properties of the SCAs need to be characterized, optimized, or
monitored, either at the standard on-orbit operating temperature (~37° K) or, during ground
testing, as a function of temperature in the vicinity of this value.
• Spatial uniformity of detector pixels, image surface flatness, row-column orthogonality,
column/row straightness [flight detector selection process; ambient conditions]
• Measure layout of FPA, SCA gap, and physical alignment
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Dark Current and Bias. Operating the SCAs without any illumination can assess this.
Reference pixels greatly reduce the need for any separate bias calibration.
Charge diffusion and electronic cross talk from naturally occurring cosmic rays in darks.
Measure full well capacity and conversion gain.
Read Noise. Evaluating the rms scatter in multiple reads of the unilluminated detector
can assess this statistical quantity.
Determine “Total noise”
Full-well capacity of the pixels, which determines the dynamic range and guides the
selection of the gain. This will normally be determined during pre-assembly testing of
the SCAs.
Linearity, particularly in the high-flux regime.
Image persistence as function of flux, time, and subsequent readouts. (Obtain at
subsystem level)
Location and properties of defective or inoperable (“bad”) pixels.
Detector response to a uniform source on several spatial scales:
o The intra-pixel response function is required to permit images that are dithered on
sub-pixel scales to be combined accurately.
o The pixel-to-pixel response function (the pixel flat field, or “pflat”) characterizes
sensitivity variations from one pixel to the next as function of wavelength over
full operating range.
o A systematic variation as a function of wavelength on larger spatial scales (the
low-frequency flat, or “lflat”) describes systematic variations on larger spatial
scales. This includes fringing that is caused by self-interference to light within
thinned layers of the detector.
Detective quantum efficiency over wavelength ranges that is superset of nominal
operating ranges; evaluate aging.
The presence and behavior of other detector artifacts; e.g., that might be analogous to the
“pedestal effect” in HST/NICMOS images can be removed with use of reference pixels.
3.10.2 Signatures to be removed from NIRSpec images
The SCAs impose a variety of signatures on the image data. All of these will be addressed in
the NIRSpec pipeline through the application of calibration reference files, which must be
developed from ground and on-orbit data.
•
•
The bias level associated with reading the detector must be removed. This will be
determined from the “reference pixels” located around the perimeter of the array,
which determine the bias level associated for each row and a left/right division of
columns. Significant variations in the bias level during the integration must also be
compensated.
The dark current accumulated during a typical maximum integration of 10 ks is
expected to be small, but will nonetheless need to be removed from the images.
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Any nonlinearities in the response of the detector to the incoming photon flux need to
be corrected to ensure the photometric fidelity of the images. These nonlinearities
will generally be in the relationship between incident and detected photons above
some flux threshold. However, nonlinearity also includes the lack of response in a
defective or otherwise inoperable pixel. Although information cannot be recovered
from the inoperable pixels, they need to be flagged.
The presence of saturated pixels must be also flagged. In some cases (e.g., pixels
saturated by a cosmic ray hit), it may be possible to recover scientifically useful
information from the affected pixel.
Pixel-to-pixel variations in sensitivity need to be corrected.
Systematic variations in detector response over larger scales (i.e., several-many
pixels) need to be corrected, including any fringing within the detector itself.
3.10.3 Mitigation Strategies
Many of the signatures imposed by the SCAs on the data can be mitigated during routine onorbit observations by the implementation of two operational strategies: multiple, non-destructive
readouts of the SCA during an integration and dithering of the observatory.
• Multiple, non-destructive “up-the-ramp” sampling of the charge collected during an
integration provides temporal information concerning the accumulation of charge in each
pixel of a SCA. In the simplest case of an unsaturated pixel, the slope of this distribution
between successive samples provides multiply redundant measurements of the photon
arrival rate. If the bias level is stable, this time-series information would not by itself
enhance the quality of the resultant estimate of the accumulated signal. However, since
the reference pixels are also monitored repeatedly, the degree to which the bias level in a
particular column (row?) is stable can be determined empirically, and significant drifts
can be detected and removed.
Up-the-ramp sampling also permits saturated pixels to be identified. In many cases,
scientifically useful information will not be recoverable, and so this identification is
primarily to inform subsequent processing of the image to ignore the affected pixels.
However, “up-the-ramp” sampling also permits cases where the saturation is caused by a
specific, one-time event (e.g., a cosmic-ray hit) to be identified. Knowledge of the
charge accumulation rate before and after such an event permits scientifically useful
information to be recovered from the pixel.
•
“Dithering” is the process of obtaining multiple, mostly overlapping images of the same
scene by re-pointing the telescope by small amounts between successive sub-exposures.
It is expected that dithering by one of a number of prescribed patterns may be used to
acquire some NIRSpec data. Subsequent processing of the images obtained at different
positions reduces the impact of detector blemishes (e.g., inoperable pixels, residual flatfield artifacts) or stationary optical ghost images, in effect by spreading them over
different regions of the field of view. For NIRSpec, sub-pixel dithering may be very
important in order to improve the sampling of the PSF, which is not Nyquist-sampled by
NIRSpec throughout much of the usable spectral range.
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The bulk of the SCA-related calibration items will require the development and maintenance of
reference files that describe the behavior of the detectors, pixel by pixel. The calibration pipeline
to remove the signatures imposed on the images by the detectors will use these; see Section 5. Of
course, the input data for nearly all these reference files must be obtained at or near the standard
operating temperature of ~37 °K.
• Read Noise: The read noise will be effectively removed by the subtraction of a
“Super Zero” exposure. The “Super Zero” is the average of many short dark exposures.
A library of these obtained with the different sampling patterns allowed for science
exposures will be maintained in a reference file. [Ground and on-orbit]
•
Dark Current: A reference file indicating the dark current generated by each
pixel per unit time (and possibly as a function of temperature around the nominal
operating conditions) is required. A preliminary version of this can be generated during
AIT, but it will have to be repeated on orbit periodically to monitor any age-related
changes. Unless there are unanticipated light leaks, dark exposures can be easily
obtained by using the provided filter wheel position that blocks the optical path to the
SCAs. Long exposures may be required. [Ground and on-orbit]
•
Linearity: The nonlinear response of each pixel to light can be characterized
during AIT by illuminating pixels with a calibrated light source. This may initially have
to be a “relative” nonlinearity, expressed, e.g., as a function of current to the lamp. The
reference file will likely take the form of a series of coefficients that fit a specified
relationship between measured signal and the linearized signal. The derived relationship
can be finalized on-orbit by observing standard astronomical sources that span a range of
known brightness levels at a few fiducial pixel locations. The “absolute” behavior at
these fiducial locations on the array could then be tied to the “relative” behavior
determined in ground tests (or on-orbit from a series of images taken at various intensity
levels of the flat-field source, if possible). This two-step approach is preferred, since the
pixel-by-pixel characterization with photometric standard stars would be prohibitively
expensive in terms of observing time. [ground and on-orbit]
•
Bad Pixel Map: Inoperable or otherwise “bad” pixels can be detected during AIT
and during on-orbit calibration activities from images obtained with a calibration lamp.
“Hot” pixels due, e.g., to cosmic-ray damage can be detected in dark exposures. [ground
and update on-orbit]
•
Intra-Pixel Response Function: Knowledge of the intra-pixel response function
is required for the most accurate combination of images dithered by sub-pixel offsets.
This information is most easily obtained by illuminating the detector pixel-by-pixel with
a highly collimated beam of light during AIT. This is an extremely time-consuming
procedure. Although it can also be accomplished on-orbit through small angle dithers, it
is prohibitively expensive to cover large sections of the detector in flight [ground and onorbit].
•
PFlats: The flat field calibration lamp(s) in the NIRSpec calibration unit can be
used to measure pixel-to-pixel sensitivity variations during AIT and on-orbit calibration
activities. Flats can also be taken at the component level, as deep exposures will be
necessary to obtain reference files with high-quality flat fields that will not degrade the
S/N of science data. During AIT OGSE continuum sources can be used to illuminate the
detector, as well. Measurement of the PFlat will be complicated by the MSA and by the
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fact that an R=3000 spectrographic configuration must illuminate the entire detector.
[Ground and on-orbit]
•
LFlats: The SCAs may also exhibit systematic variations in their response to
uniform illumination over scales that range from several to dozens of pixels. Fringing
caused by interference effects in the surface layers of the detector is one example of this
behavior. These variations are often difficult to characterize accurately during AIT, since
they can depend very sensitively on the input beam from the GSE, which may not mimic
the on-orbit performance of the OTE with high fidelity. Unfortunately, they are also
difficult to characterize on-orbit, because it is hard to find an astronomical source with
uniform surface brightness. Additional detailed information pertaining to Lflat strategies
and issues are found in NIRS-CRAL-TN-0004. [Ground and on-orbit]
3.10.4 Remarks
Characterization and verification of the properties of the SCAs must be accomplished both on the
ground in AIT and on a regular basis in flight. ECF ground-calibration concept recommends
characterization of spatial uniformity characteristics at ambient and balance of testing anticipated
operating temperatures. Accuracies of flat fielding and fringing calibrations depend on
observing strategies implemented in flight, such that requirements should be reviewed
periodically as development of observation methods proceeds.
Anticipated testing environment: Component, system (unit) level, on-orbit commissioning,
on-orbit routine calibration
Now that we have listed a variety of calibrations appropriate at the component level, we turn to
those types of calibration that must be obtained at the unit, system, DM/ETU, or FM level on an
end-to-end basis for the NIRSpec instrument in imaging mode, multi-object spectroscopy mode,
fixed-slit mode, and integral-field spectroscopy mode. A number of the calibrations performed
at the component level must be repeated at the integrated system level as characteristics may
change in the unit environment. Many of these calibration procedures will produce data that can
be used to generate the initial set of calibration reference files, which will be required by the data
processing pipeline.
Many of the following system level tests also may be performed on-orbit with a suitable
astronomical target or with an appropriate internal calibration channel mode.
The calibrations and characterizations listed in the rest of this section for the greater part derive
from the ground calibration plan of P. Ferruit (NIRS-CRAL-PL-0001) and descriptions in the ECF
ground calibration concept document. As such this listing is intended to capture the essence of
those documents.
3.11 Optical Performance/Alignment
3.11.1 Scattered or stray light
Measure the amount of scattered light for all operating modes including those using the
internal calibration lamps. Determine level of stray light due to PSF wings, back
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scattering off MSA structures, filters, and other internal structures, and determine
location and strength of any ghost images. ECF calibration concept recommends
performing all tests at nominal ISIM temperature.
3.11.2 Focus and PSF-related calibrations
Determine the best focus and two-dimensional PSF in FOV as function of wavelength for
both spectroscopic and imaging modes; characterize variations of PSF with deviation
from best focus; determine variation of PSF over entire FOV. Measure the PSF at the
AFP and the detector with and without MSA and slits. ECF calibration concept
recommends performing all tests at nominal ISIM temperature.
3.11.3 Positional alignment calibrations
Demonstrate and verify any positional calibration procedures required for on-orbit
mechanism alignments and aperture locations. ECF calibration concept recommends
performing all tests at nominal ISIM temperature.
3.11.4 Filter offsets
Measure spatial offsets introduced by filters relative to clear filter wheel position.
ECF calibration concept recommends performing all tests at nominal ISIM temperature.
3.12 Geometric Characteristics and Location of Images
3.12.1 OTE-FPA distortion
Utilize OTE simulator with pinhole grid mask with mirror mode. Combine with MSAFPA distortion map to obtain full OTE-MSA distortion solution. ECF calibration
concept recommends performing all tests at nominal operating temperature.
3.12.2 MSA-FPA distortion
Utilize continuum lamp and variety of MSA patterns. Combine with MSA-FPA
distortion map to obtain full OTE-MSA distortion solution. ECF calibration concept
recommends performing all tests at nominal operating temperature.
3.12.3 Spectral trace
Utilize internal calibration channel lamps to measure the positioning and orientation of
the dispersed light spectrum trace from each disperser for all fixed slits and as a function
of utilized positions with the MSA. Evaluate trace line tilts and orientations as a
function of wavelength for all dispersers. Locate and evaluate detector gaps. ECF
calibration concept recommends performing all tests at nominal operating temperature.
3.12.4 Shape and position of slits
Measure the projection of MSA slits on detectors and relative position of fixed slits to
facilitate target acquisition. Evaluate influence of detector gaps. ECF calibration concept
recommends performing all tests at nominal operating temperature.
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3.13 Target Acquisition
The target acquisition algorithms must be verified. The influence of geometrical distortion on
TA must be evaluated. The positions measured by the TA algorithm should be determined as a
function of real position. The influence of grating wheel instability on TA accuracy should be
evaluated.
3.14 Internal Calibration Channel
3.14.1 Lamp illumination
Measure the illumination uniformity of all line and continuum lamps. ECF calibration
concept recommends performing all tests at nominal operating temperature and possibly
bracketing temperatures.
3.14.2 Applicability of line lamps
Perform detailed comparison of external OTE simulator calibration lamps with internal
calibration channel lamps for all operating modes to validate the internal unit. ECF
calibration concept recommends performing all tests at nominal operating temperature
and possibly bracketing temperatures.
3.14.3 SNR as function of exposure time
Measure the SNR versus exposure time for all lamps in all operating modes. Perform
lamp-heating tests. Identify all internal calibration channel modes that could affect safety
of detector or impact observations subsequent to the calibration exposures (e.g., image
persistence). ECF calibration concept recommends performing all tests at nominal
operating temperature and possibly bracketing temperatures.
3.14.4 Evaluate line sources
Measure the wavelength and line strength of all line features over the nominal operating
wavelength range of each disperser and operating mode. Highest resolution observations
may be needed to resolve blends. ECF calibration concept recommends performing all
tests at nominal operating temperature and possibly bracketing temperatures.
3.15 Mechanism Motion and Repeatability
The positional repeatability and mechanism stability of the several mechanical elements present
in the optical path that can affect observational quality must be evaluated at all stages of
integration through flight operation. These elements include the filter wheel, the focusing
mechanisms, and, of particular importance for spectroscopic operation, the grating wheel.
Characterization should occur at ambient and at mean ISIM temperature.
3.16 Spatial Uniformity and Photometric Calibrations
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In the following, for system photometric throughput measures utilize a source of known
radiometric output with system OTE simulator to measure absolute throughput in each band.
Photometric throughput should be measured for all imaging and spectroscopic modes as
appropriate to characterize each of the following:
• Shutter throughput
• Slit throughput
• IFU throughput
• System photometric response: MSA
Measure point source throughput at a grid of positions within an MSA element;
repeat at a series of locations across MSA grid. ECF calibration concept
recommends performing all tests at nominal ISIM temperature.
• System photometric response: fixed slits
• System photometric response: IFU
• System photometric response: filters
• System photometric response: dispersers
Use OTE source to obtain flux-calibrated zeroth, first, and second order spectra
for several positions within the FOV.
• Background
Flat fields can be obtained with external OGSE continuum sources and with the internal
calibration channel. The determination of a full pixel-by-pixel flat field as a function of
wavelength would be extremely costly on-orbit; therefore, high-quality and detailed groundbased flat fields are extremely important.
• Small and large scale flat fields
Use continuum lamp and OTE simulator to determine flat field for each filter and
disperser over entire FOV. These flats will be used for the instrument model.
ECF calibration concept recommends performing all tests at nominal ISIM
temperature.
3.17 Spectroscopic and Wavelength Calibrations
3.17.1 Dispersion solutions
Measure the detector position-wavelength relation for all spectral modes with input from
the OTE simulator over the entire FOV. Measure the detector position-wavelength
relation for all spectral modes with the internal calibration channel. Locate the positions
of detector gaps. The data obtained from the external source will serve as input for the
instrument model and will calibrate the internal channel and establish initial wavelength
reference data. ECF calibration concept recommends performing all tests over a range of
nominal operating temperatures.
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3.17.2 Wavelength zero points
Measure the dispersed light zero points as a function of operating temperature, operating
sequences, and instrument configurations. ECF calibration concept recommends
performing all tests over a range of nominal operating temperatures.
3.17.3 Line Spread Function
Measure the line spread function for all spectroscopic modes and slits with both the
internal calibration channel line lamps and with external sources.
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3.18 Integral Field Unit Calibration
Although the IFU shares the same detectors as the MSA and fixed slit modes of operation, the
unit is sufficiently different that its calibration should be specifically enumerated. Many of the
calibration types described above apply to the IFU and, indeed, the IFU has been mentioned in
many of those descriptions, however they are again listed below for completeness. Unless
otherwise noted, all of the following should be performed at nominal NIRSpec operating
temperature.
3.18.1 Throughput
Measure the throughput of the whole instrument with each of the dispersers when used in
IFU mode. Determine absolute throughput at center of FOV and relative throughput for
every pixel of the FOV.
3.18.2 Contrast
With an appropriately sized source centered on one slitlet, measure the scattered or stray
light on all other slitlets; perform this measurement for all slitlets and dispersers.
3.18.3 Flat Field
Measure both spatial and wavelength dependence of the flat field when using the IFU
with each disperser.
3.18.4 Focus
Determine best focus for IFU. Perform at temperatures bracketing nominal operating
temperature in order to assist on-orbit calibration, especially in the cool down period.
3.18.5 Spatial PSF
Obtain re-constructed images of objects located at many positions covering the entire IFU
FOV to characterize the spatial PSF variations over the FOV.
3.18.6 Spectral PSF
Obtain line profiles at many source locations to provide the LSF for all field points within
the IFU FOV. If possible, utilize fine stepping of the gratings to move features by
fraction of a pixel to facilitate sampling of the LSF. Accurate knowledge of the LSF is
important for IFU background subtraction procedures.
3.18.7 Wavelength Calibration and Spectral Trace
Obtain wavelength calibration mapping for all dispersers at many positions covering the
entire IFU FOV. Characterization of zero point drifts, as a function of operating
temperature should be performed. Evaluate trace positions and, especially, line tilts and
orientations as a function of wavelength for all dispersers.
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3.18.8 Slitlet Positions
Determine center-to-center distances in pixel units between slitlets of the image slicer,
which is vital for reconstruction of images from the dispersed spectra. This
measurement, which should be performed with all dispersers, will require an external
continuum source. Slitlet positions should be characterized over the entire operating
wavelength range. Reference files will be prepared from this information.
3.18.9 Optical distortion
Optical distortion introduced by the IFU will be different from that introduced by the
MOS and slit observing modes. Distortion measurements for the IFU modes are
required.
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Assessment of On-Orbit Calibration Needs
This section covers on-orbit calibration activities for NIRSpec including those to be performed in
both the verification or commissioning period and in routine science operations. At the present
time, the routine science calibration portion of this section is more detailed, but descriptions of
activities for both periods are early drafts and will require more definition as the details of
pipeline processing and ground-based instrument calibration are refined.
Since calibration activities during the post-launch phase of the JWST mission carry a price in
terms of observing time or science efficiency, an initial suite of calibration reference files and
knowledge of other parameters required to optimize instrument performance must be obtained
during AIT.
Ground testing of an assembled instrument is typically limited by the difficulty in simulating the
true JWST operating conditions, particularly with regard to the space environment (temperature,
vacuum, radiation) and the input to the optical system. As a result, high-fidelity calibration of
the performance characteristics of the assembled instrument generally occur during the
commissioning phase, and subsequently during efficient, regularly scheduled monitoring of key
parameters. These observations are often aided by the availability of astronomical sources that
are brighter or otherwise better suited to rapid observation.
The aspects of NIRSpec calibration that will particularly benefit from on-orbit characterization
are:
• Astrometric calibration: optical distortion in the camera
• Flat Field calibration (pflats and lflats) – may require modeling input drawn from groundbased calibrations
• Photometric calibration: conversion of raw detector counts to relative and absolute fluxes
• Wavelength calibration
• Measuring the instrumental background due to stray and scattered light
• Measure SCA characteristics in actual flight environment
The following calibration-related activities also have to be performed on-orbit, particularly in the
commissioning period:
• Target acquisition verification and accuracies
• Aperture location
4.1.1 Commissioning Period Activities
The following NIRSpec functional requirements and those in sections 4.1
through 4.5 are relevant to SI commissioning:
NSFR-3 All requirements in this section shall be fulfilled at operational environmental
conditions at Second Lagrange (L2) and for all operational modes independent of previous mode
except when mentioned otherwise.
NSFR-35 NIRSpec shall support an autonomous target acquisition process.
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NSFR-36 The target acquisition procedure shall autonomously determine the required spacecraft
slew (pitch, yaw and roll) such that the centroids of up to 20 acquisition targets acquire predetermined positions in the MSA focal plane.
NSFR-37 The target acquisition procedure shall enable placement of all science targets in the
MSA plane to an accuracy of better than 12.5 mas (1 sigma, in each x and y direction) over at
least 104 s with no more than 20 reference stars.
NSFR-38 Target acquisition for a new target field shall be accomplished within 10 minutes.
NSFR-42 In R=100 mode, the optical throughput at BOL of the NIRSpec optics, including the
R=100 dispersing element and any required order blocking filter shall be ≥ 60% for the spectral
range between 0.6 and 5.0 µm and for any position in the FOV.
NSFR-43 In R=1000 mode, the average optical throughput at BOL of the NIRSpec optics in all
the three spectral bands, including the relevant R=1000 dispersive element and the relevant order
blocking filters, shall be ≥ 45% for the spectral range between 1.0 µm and 5.0 µm and for any
position in the FOV.
NSFR-44 In R=3000 mode, the average optical throughput at BOL of the NIRSpec optics in all
the three spectral bands, including the relevant R=3000 dispersive element and the relevant order
blocking filters, shall be ≥ 40% for any position in the FOV.
NSFR-45 The overall performance degradation regarding throughput of NIRSpec between BOL
and EOL shall be less than 10%.
NSFR-49 The mean detective quantum efficiency (DQE) of the NIRSpec SCAs (averaged over
all operable pixels) shall be higher than 80% for all wavelengths between 1 µm and 5µm, and
higher than 70% for all wavelengths between 0.6 µm and 1µm.
NSFR-50 The pixel-to-pixel variation of the DQE shall be less than 10% root mean square
(RMS) when measured over all operable pixels.
NSFR-51 The NIRSpec detector system including FPA, FPE and harness shall have a mean total
noise per pixel of less than 6 e-, where the mean is computed over all operable pixels.
NSFR-52 The NIRSpec SCAs shall have a well capacity of at least 60,000 e- per pixel, averaged
over all operable pixels.
NSFR-39 NIRSpec shall obtain low resolution (R=100) spectra over the wavelength range 0.6
µm - 5.0 µm in a single exposure.
NSFR-40 NIRSpec shall obtain medium (R=1000) and high (R=3000) resolution spectra over
the wavelength range 1.0 µm - 5.0 µm in no more than three separate exposures.
NSFR-41 In R=100 mode, the spectral resolution over the spectral range 1.0 µm – 5.0 µm shall
not exceed the range R = 50 - 200 for a 200 mas wide slit or MSA shutter.
NSFR-55 The attenuation factor of light that enters the NIRSpec while the filter wheel is in the
“closed” position shall be better than 106, i.e. less than 1 part in 106 of light shall reach the
NIRSpec SCA. This requirement is applicable for any position of the source with respect to
NIRSpec.
NSFR-56 NIRSpec shall not increase the straylight levels entering its FOV as specified in the
James Webb Space Telescope Project Mission Requirements Document (JWST-RQMT-000634,
MR-121 and MR-122).
NSFR-57 The total signal of in-field light that is not focused into the primary source image
(“ghosts”) shall be less than 0.5% of that of the primary image.
NSFR-58 The flux transmitted through an MSA shutter in the “closed” state shall be less than
1/2000 of the flux transmitted through a shutter in the “open” state.
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Before genuine on-orbit calibration activities can begin, the NIRSpec will have to undergo a
suite of functional tests in order to establish, e.g., whether the grating and filter wheels and other
mechanisms respond correctly to commands; whether the calibration sources work; and so on.
Meaningful instrumental verification or external scientific calibration of the NIRSpec cannot
begin before the nominal operating temperature is reached and the wave-front sensing and
control (WFS&C) process has converged to provide a stable point-spread function. Initially a
series of alignment activities and aperture locations will be performed. Subsequent to these basic
alignments more science-oriented verifications and characterizations will be performed.
Essentially most of the activities enumerated in ground calibration sections 3.11 through 3.20 are
initial candidates for inclusion in the commissioning period calibrations and should be evaluated
carefully due to limitations of observing time during this commissioning period. An initial
estimate of the priorities will be to:
• Perform initial SI optics alignments and locate apertures for all modes.
• Verify autonomous target acquisition procedures and determine offsets
• Obtain full-field images of designated astronomical objects in order to characterize the
optical distortions produced by the NIRSpec; see Section 4.1. Since it is likely that the
other SIs will adopt the same “astrometric” fields, this activity will be conducted jointly
with them. This imagery will also be used to refine aperture locations and TA offsets and
verifies alignments at nominal operating temperature.
• Confirm the ground-based wavelength calibration; see Section 4.4
• Confirm spectral resolution
• Observe photometric standards to establish transformations between the instrumental
magnitude system and a calibrated flux scale. See Section 4.3. This initial set of
observations is of particular importance since it will set the baseline for subsequent
monitoring.
• Obtain a library of dark exposures under genuine operating conditions.
• Obtain a library of flat-field exposures under genuine operating conditions; see Section
4.2.
• Confirm the optimal detector parameters (gain, read-out noise, bad-pixel maps)
determined during ground-test.
• Obtain some deep exposures of “blank” sky to begin to characterize the stray and
scattered light contributions to the observed background.
• Evaluate on-orbit performance of optimal dithering strategies.
• Evaluate the influence of telescope re-phasing on focus and various calibration-related
quantities such as distortion, PSFs, and LSFs.
4.1.2
Science Operations Era Calibration Activities
STScI is constructing a science operations design reference mission (SODRM that includes
routine calibrations for the science instruments that will be performed during the science
operations period. External calibrations use primary time with the telescope. Here we include all
of the presently proposed NIRSpec calibration monitoring programs for the SODRM. These
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programs are meant to represent the typical calibration programs for NIRSpec after it has been
commissioned. Commissioning activities will of course be longer and more involved. Short
text descriptions of the present set of calibration monitoring programs listed below are in
Appendix A.
•
•
•
•
•
•
650
651
652
653
660
661
NIRSpec Dark Monitor
NIRSpec Flat Field Monitor
NIRSpec MSA to Detector Distortion Monitor
NIRSpec Wavelength Monitor
NIRSpec Spectrophotometric Throughput Monitor
NIRSpec Sky to MSA Distortion Monitor (Astrometric Monitoring)
Additional possible programs not yet in the SODRM:
•
•
xxx NIRSpec Frequent Throughput Monitor (Contamination Monitor)
xxx NIRSpec Stray and Scattered Light Characterization
Updated short text descriptions of these programs are available at:
http://www.ess.stsci.edu/projects/sodrm/drmprop/prop.html
4.2 Astrometric Calibration and Image Quality
Relevant FRD requirements:
NSFR-46 NIRSpec shall be shall have the same axial focus position independent of grating and
filter selection.
NSFR-47 Assuming the above OTE WFE, the NIRSpec Fore Optics shall provide an image on
the MSA focal plane that is diffraction-limited, i.e. has a Strehl Ratio > 0.80, at a wavelength of
2.46 µm (TBR 4) for any position in the FOV. This requirement is equivalent to limiting the
WFE to less than 185 nm.
NSFR-48 Assuming the above OTE WFE, the OTE optics and NIRSpec optics shall provide an
image at the focal plane that is diffraction-limited, i.e. has a Strehl Ratio > 0.80 (WFE < 238
nm), at a wavelength of 3.17 µm over the entire optically exposed active area of the FPA.
NSFR-73 Assuming the above OTE WFE, the OTE optics and NIRSpec optics shall provide,
when the IFU is in use, an image at the focal plane that is diffraction limited, i.e. has a Strehl
Ratio > 0.80 (WFE < 258 nm), at a wavelength of 3.44 µm over the entire optically exposed
active area of the FPA.
The goal of astrometric calibration of images obtained with the NIRSpec is to convert the
positions of sources located at specific pixel positions on the SCAs to positions on the sky. This
conversion is motivated by the desire to (a) measure positions accurately; and (b) use the
measured positions of the same objects in different images to combine the images in a way that
maintains the integrity of the information. A particular application of (b) is the merging of
dithered images of the same scene. In the first instance, accurate relative positions of objects are
required. Absolute positions can subsequently be obtained by tying the position of one or more
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sources in the field to an established frame of reference (e.g., J2000). Insofar as these
manipulations must remove the geometric distortions imposed on the image by the optical
system, a frequent by-product is the regularization of the solid angle of the sky subtended by
each pixel in the SCA.
The basic approach to accurate astrometric calibration has been established by Anderson & King
(2000, PASP, 112, 1360 and 2003, PASP, 115, 113). The first step is to remove the systematic
errors in position in a single image caused by the random placement of a stellar image with
respect to the grid of pixels that comprises the SCA. This is accomplished by using the images
of many stars distributed across the field of view and sampled in many different ways to derive
an extremely accurate PSF. Successive iterations use improved estimates of the PSF to return
revised measurements of the positions; the procedure generally converges rapidly. The second
step involves combining the positions of objects on different images, which requires
transformations between the coordinate systems of images that are translated or rotated with
respect to each other. These transformations will typically be nonlinear, owing to the distortions
(or residual distortions) imprinted by the optical system, as well as possible time-dependent scale
changes. However, with sufficient numbers of objects in overlapping images, the
transformations can be determined reliably. In ideal cases, an astronomical image contains
many (~105) point sources, and this dense sampling permits a refined determination of the image
distortion (residual or otherwise) compared with that available from ground tests. A by-product
will be an accurate astrometric calibration of the NIRSpec.
Globular clusters or dense open clusters are the optimal objects for this analysis, since they
provide many point sources over the field of view of the NIRSpec. The basic strategy is to
observe the same field and multiple roll angles – perhaps, e.g., ~6 months apart, during which
time the FOV will have rolled by ~180° – so that most the images of most stars will be found on
very different parts of the SCA. However, many of the objects that have been studied
extensively, e.g., with HST instruments, contain stars that are bright enough to saturate the
JWST detectors. A Working Group under the leadership of Dr. James Rhoads (STScI) is
currently studying suitable fields for astrometric calibration in the Large Magellanic Cloud,
which has the added advantage of being in the continuous viewing zone (CVZ) for JWST, and
hence available for study at regular intervals. It is desirable to use a common field for the
astrometric calibration of all JWST SIs. Further efficiencies could be achieved if the same field
could be used simultaneously for photometric calibration. Astrometric calibration of the IFUs
may be achieved by raster scans of a bright, isolated point source. High order astrometric
distortion terms are expected to be small for the IFUs because of their small field of view, and
they will be characterized in ground calibration (section 3.19.9).
An outstanding issue is to establish the frequency with which astrometric calibration should be
repeated. Once normal operating temperatures are achieved, the aging of components may
produce slow changes in the distortion corrections. From this perspective, a full astrometric
calibration observation probably needs to be repeated only when large deviations are detected in
routine examination of science images. However, if the PSF delivered to the science instruments
changes significantly when the OTE is reconfigured, then astrometric calibrations might need to
be performed more frequently.
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An additional concern is whether the instrument distortion changes as a function of
spacecraft orientation, perhaps because slight flexure in the OTE changes the light beam that
feeds the science instruments. If the astrometric calibrations obtained from observations in the
LMC (i.e., in the CVZ; near the orbital pole) are not sufficient, a family of astrometric
calibrations corresponding to different attitudes might have to be developed.
4.3 Flat-Field Calibration (Differential Photometric Calibration)
Relevant ESA SRD requirement:
R 176: In orbit, NIRSpec shall enable the calibration of the pixel-to-pixel response maps as a
function of wavelength and to an accuracy of 2% or better when applying the on-board
continuum calibration source. This requirement is valid for the complete NIRSpec FOV and for
all the exposed wavelengths. NOTE: This calibration must measure dispersed flat fields at the
FPA.
As discussed in Section 3.10.3, pixel-to-pixel sensitivity variations in the SCAs will be removed
by observations of the flat-field lamps (“pflats”) in the onboard calibration unit. Certain other
variations that may occur on larger spatial scales (e.g., fringing) will also be removed through
observations of the lamps (“lflats”).
However, it may be that other systematic variations in sensitivity exist on multi-pixel scales. It is
conceivable that these variations might not be adequately represented in the “lflats”, e.g.,
because the illumination of the detector is not sufficiently similar between observations of
astronomical sources and the onboard lamps. In this case, “sky-flats” could be constructed, e.g.,
by long-duration observations of zodiacal light or other carefully selected, nearly uniformly
bright sources. This approach would be very time consuming and would be practicable for only
a few setups. Even then, it would only be feasible if the sky flats were stable on time scales of a
few months, or if parallel observations were possible. Spectrophotometric standard stars may be
observed at a few locations in the field of view to determine the response and combined with
ground “lflats” – this procedure has the advantage of not de-coupling absolute and relative
radiometric calibrations. If moving target capability is developed, then a bright standard could
effectively be trailed through along a slit or through portions of the FOV.
Flat fields will be obtained as part of routine science calibration activities approximately 3-4
times per year. An important consideration for on-orbit flat fielding will be the precise set of
exposures deemed necessary to provide the required characterization as a function of
wavelength. Will separate flats be needed for each disperser and spectrum band or will
wavelength characterization alone be sufficient? The exact distribution of exposures and grating
or filter positions to be used will be determined as part of ground testing.
4.4 Absolute Photometric Calibrations
Relevant FRD requirements:
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NSFR-14 After on-ground and in-orbit calibration, the absolute photometric accuracy of all
NIRSpec science data shall be better than 10%.
NSFR-9 In R=1000 mode, using a single shutter of the MSA or a 200 milli-arcseconds (mas)
wide fixed slit, NIRSpec shall be capable of measuring the flux in an unresolved emission line of
F = 5.2 x 10–22 Wm-2 from a point source at an observed wavelength of 2 µm at signal to noise
ration (SNR)=10 per resolution element in a total exposure time of 105 seconds (s) or less.
NSFR-10 In R=100 mode, using a single shutter of the MSA or a 200 mas wide fixed slit,
NIRSpec shall be capable of measuring the continuum flux of Fν = 1.2x10-33 Wm-2 Hertz (Hz)-1
from a point source at an observed wavelength of 3 µm at SNR=10 per resolution element in a
total exposure time of 104 s or less.
Photometric calibration is required to convert the flux of an astronomical source as measured in
the arbitrary, instrumental system of the NIRSpec into standardized physical units, so that they
can be compared with other observations or theoretical calculations. Although the sensitivity of
an instrument can characterized quite well during AIT, this final step of deriving absolute
conversion factors is best left until on-orbit commissioning, as no true end-to-end photometric
calibration can be established before flight as it is not possible to have a system level calibration
with NIRSpec and the JWST telescope system together. The conversion is typically
accomplished by observing a suite of astronomical objects, preferably point sources (i.e., stars),
with known brightness in a well-defined waveband. By comparing the observed signals (in
ADU/s) of these photometric standards with the known fluxes (e.g., in Jy), the conversion can be
determined empirically. For the broadband filters, color terms may also be required by this
absolute photometric calibration. As stated in NSFR-9, the absolute calibration of NIRSpec
science data shall be better than 10%.
It is highly desirable that the photometric calibrators be drawn from different populations of stars
that cover a wide range of flux levels as similar as possible to typical NIRSpec science usage, so
that (a) systematic errors associated with the calibration of any one type of object do not
influence the calibration of NIRSpec; (b) the effects of color terms can be explored; and (c) the
efficacy of the linearity correction can be checked and other systematic effects that might depend
on the brightness of the source can be investigated. The types of objects that will be particularly
useful for calibrating the NIRSpec photometrically are:
• DA white dwarf stars which have a long heritage as HST spectroscopic standards and
which have relatively simple atmospheres of nearly pure H that can be well
characterized with non-LTE model atmospheres. In particular, most of the
HST/NICMOS standards discussed by Bohlin, Dickinson, & Calzetti (2001, AJ, 122,
2118) will be accessible to NIRSpec. Rapid, sub-array readout may be required to
observe some of the brighter objects.
• Solar analogs, likely the same objects that may selected for the photometric
calibration of NIRCam, can be used to derive absolute fluxes through comparison
with the spectrum of the sun. This approach has been developed by Campins, Rieke,
& Lebofsky (1985, AJ, 90, 896) and used to calibrate e.g., HST/NICMOS and
Spitzer/MIPS. The accuracy of the method ultimately rests on the degree to which a
targeted G-type dwarf is truly a solar analog.
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Since they have considerable heritage as photometric calibrators across the electromagnetic
spectrum, the white dwarf stars used by HST/NICMOS, in particular G191B2B, GD71, and
GD153, will be the primary calibrators; see Figure 3. Supplemental observations of solar-type
stars will permit reliable color corrections to be derived, and will help to define the linearity of
the flux calibration at its fainter end. Faint white dwarfs drawn from the enormous population
of candidates recently detected in the globular cluster ω Centauri by Monelli et al. (2005, ApJ,
621, L117) might also serve to define the faint end of the calibration, providing suitable targets
can be identified in the outskirts of the cluster.
Figure 3: HST standard stars suitable for photometric calibration of NIRSpec;
Figure 3 is from Bohlin et al (2001, AJ, 122, 2118).
At least one of a subset of ~3 primary standards will have to be observed frequently through the
life of the mission, in order to monitor photometric degradation of the NIRSpec. These
standards will need to be chosen to ensure availability at any time of the observing year,
although even then it is advisable to rely on one primary calibrator as much as possible.
4.5
Wavelength Calibration
Relevant FRD requirements:
NSFR-15 After calibration, the wavelength scale of NIRSpec spectra shall be determined with
an accuracy of better than 1/8 of a spectral resolution element.
The relationship between pixel coordinate and wavelength is determined by observation of a
suitable source of spectral features of known wavelength. A polynomial is fit to the set of
measured line centers and known wavelengths. Onboard calibration sources will be chosen to
provide comprehensive coverage of as many spectral regions as possible. The frequency with
which this calibration needs to be checked will depend primarily on the stability and repeatability
of the grating wheel mechanism that is used to place the gratings in the optical path. In most
cases, it likely will be desirable to obtain a wavelength calibration exposure immediately before
or after a science observation with the same setup, in order to determine the exact value of the
central wavelength during subsequent processing of the image.
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Since the optical path for the internal calibration channel lamps is not identical to the optical path
for external sources and since the density of bright lines emitted by the calibration lamps may not
be sufficient for primary calibration of all possible wavelength settings, observations of
astronomical sources may be utilized to establish internal-to-external offsets as well as the
dispersion relation for certain instrumental setups.
Planetary nebulae are excellent sources of dense grids of bright emission lines. NGC 6543, the
“Cat’s Eye Nebula,” may be particularly suitable, since it frequently will be located in the
northern-CVZ for JWST.
4.6 Calibration of Stray and Scattered Light
Relevant FRD requirements:
NSFR-54 The NIRSpec optics shall be sufficiently oversized so that no more than 10% of the
light passing through the slit is lost due to diffraction effects. This applies to all wavelengths
between 0.6 µm and 5.0 µm and all positions within the NIRSpec FOV.
NSFR-55 The attenuation factor of light that enters the NIRSpec while the filter wheel is in the
“closed” position shall be better than 106, i.e. less than 1 part in 106 of light shall reach the
NIRSpec SCA. This requirement is applicable for any position of the source with respect to
NIRSpec.
NSFR-56 NIRSpec shall not increase the straylight levels entering its FOV as specified in the
James Webb Space Telescope Project Mission Requirements Document (JWST-RQMT-000634,
MR-121 and MR-122).
NSFR-57 The total signal of in-field light that is not focused into the primary source image
(“ghosts”) shall be less than 0.5% of that of the primary image.
NSFR-58 The flux transmitted through an MSA shutter in the “closed” state shall be less than
1/2000 of the flux transmitted through a shutter in the “open” state.
NSFR-8 NIRSpec shall have a dedicated aperture for high-contrast, single-object spectroscopy
with an out-of-slit rejection ratio higher than 106, i.e. less than 1 part in 106 shall be transmitted
outside of the slit.
Stray and scattered light is difficult to characterize prior to orbital operations for two reasons.
First, typical OGSE may not have sufficient fidelity to obtain meaningful measurements of
scattered light. Second, and more fundamentally, it is generally not possible to distinguish
scattered or stray light from the dark current intrinsic to the operation of the detector. Since
these two quantities are only determined together, the strategy is to use on-orbit observations of
different astronomical sources to disentangle them. For example, comparison of “dark”
observations (i.e., observations obtained with the filter wheel in its blocking position in the light
path into the SCA) with the same instrumental setup when JWST is pointing at (a) a bright
source and (b) a faint piece of “blank” sky at high ecliptic latitude should permit the gross
characteristics of the stray/scattered light to be determined.
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Draft Science Calibration Pipeline
For most practical purposes, the calibrations described in the previous sections will be applied to
the images obtained with the NIRSpec during the course of routine post-observation processing.
This processing will make use of reference files that quantify different properties of the
instrument in a way that facilitates automatic removal of their effects on images. The end result
will be a 2-d image and, for spectroscopy, an extracted spectrum (or several extracted spectra) or
a series of combined images and extracted spectra (or several extracted spectra) suitable for
scientific measurement and interpretation
5.1 Overview of Processing for Data from JWST Science Instruments
The processing of science data from satellite observatories is conventionally described in terms
of the stages or “levels” through which it must pass. The flow proceeds as follows:
•
Level 0: Communications decommutation
This step creates ordered sets of error-corrected, compressed data packets.
•
Level 1a: Format Conversion
This step removes the packet formats, decompresses the data, determines which packets
belong to specific observation sets, and recompresses the data in a format suitable for the
telemetry archive.
•
Level 1b: Generic Conversion
Level 1b processing decompresses the science data, performs validation, and converts it
to the standard Flexible Image Transport System (FITS) format. A FITS header is
created and initial values of header keywords are populated based on information
associated with the observation or information contained in the Project Reference
Database (PRD). For example, information about the scheduling of the observation,
requested and actual instrument configuration, detailed pointing (including
transformations to World Coordinate System), and preferred reference files are
incorporated into the headers at this stage.
•
Level 2: Calibration Processing
Level 2 processing performs the “scientific” calibration of individual exposures through
the application of algorithms and reference file that are specific to each instrument. In
this step, the instrumental signatures are removed from the raw images, and conversions
to physical units are either implemented or provided in the headers for rapid conversion.
The headers are updated with information that describes the processing (which steps were
completed successfully, which reference files were used, etc.) and quick-look images are
produced to enable on-line browsing of the archived data. In some circumstances, a
series of images of the same object that were obtained as part of a pattern of dithers may
be combined at this stage, though this would typically occur in the next step.
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Level 3: Association Processing
Level 3 processing comprises the final step of the manipulations routinely
performed at the S&OC. It combines “associated” sequences of dithered images of the
same field, which in the case of NIRSpec, will likely be used to enhance the sampling of
the PSF and the removal of residual flat-field artifacts. Catalogs of objects in each
dithered image will likely be created routinely at this stage
Level 0 processing will typically performed at the ground station, though to meet contingencies
this capability will also reside at the Science & Operations Center (S&OC) at STScI. All
subsequent processing steps occur at the S&OC. The Multimission Archive at Space Telescope
(MAST), which is also hosted at STScI, will be provide long-term storage of the data, as well as
interfaces for query and retrieval of the data by the user community.
5.2
File Formats
During Level 1 processing, data from the JWST SIs will be converted to FITS format.
The trade study led by Dr. Jerry Kriss (STScI-JWST-TM-2004-0025) found that the optimal
structure for raw data files is:
• There should be one FITS file for each exposure and SCA.
• MULTIACCUM exposures should be stored as a data cube in the primary data unit
(PDU) of the FITS file. This cube is given by (x, y, z), where x and y represent pixel
locations, with x, y∈(0, 2047) and where z is the sample number, z = 1,…,Ngroup frames.
The “z” dimension of the data cube can also be thought of as the time axis. This format
is particularly conducive to “up-the-ramp” fitting to determine the photon arrival rate,
since a projection along the z-axis for any given (x, y) pixel location provides a complete
history of the accumulation of signal on that pixel.
• Ancillary information appropriate to the entire exposure should be stored in header
keywords in the PDU.
• Engineering data describing the circumstances associated with each of the Ngroup frames
should be stored in the first extension of the FITS file as a binary table.
The output of subsequent processing steps will also be stored in FITS format, whose names will
be altered to reflect the manipulations that have occurred and to protect the integrity of the input
files. Changes to the files will be noted by updating the appropriate header keywords, e.g., to
indicate which processing steps had been performed and perhaps to record salient output from
the processing. At various stages the structure of the image data will also change. For example,
the “data cube” representation of the MULTIACCUM sampling will collapse to a single twodimensional image following the pixel-by-pixel determination of the count rate via “up-theramp” fitting. Information about the processing will also be available in a “trailer file” that lists
information pertinent to reviewers interested in the quality of the data product. The intermediate
and final output products of Level 2 and 3 processing may have unique structures, e.g., processed
IFU data will likely be in the form of a data cube. These product file structures should be the
subject of future calibration working group discussions.
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Pipeline Structure
The NIRSpec IDT plan incorporated in following for which some important issues to be resolved
are:
• Should there be separate pipelines for imaging and spectroscopy modes? At present
this discussion assumes pipeline operation is the same to a certain critical point in the
processing for part A. A separate “-imag” and “-spec” pipeline branch is based on
mode keywords for part B.
• Should there be differences for IFU versus slits / MSA?
• Should there be processing for slitless spectroscopy?
• The file structure of output data products, especially for multiple extracted sources
and for IFU observations must be determined
As an initial attempt at organization, we propose that datasets and associations produced by the
NIRSpec will be calibrated by the calNIRSpecA and calNIRSpecB tasks, respectively.
The main steps performed by the pipeline are:
1. The data from JWST are transmitted to Earth partitioned into separate
engineering and science telemetry. The first step is to associate the
science data with the appropriate engineering telemetry.
2. The data are edited, if necessary, to insert fill values in place of missing
data.
3. The data are evaluated to determine if there are discrepancies between
a subset of the planned and executed observational parameters.
4. A list of calibration reference files to be used in the calibration of the
data is created based on the executed observational parameters.
5. The raw data are converted to a generic (FITS) format and the header
keyword values are populated (known as generic conversion).
6. Depending upon mode and SI setup, the raw data are calibrated using a
standard process, calNIRSpecA, described below, to remove the
instrumental signature. calNIRSpecA is performed on each SCA
individually on the MULTIACCUM data following each individual detector
reset.
7. The calNIRSpecA -calibrated data may be combined into an image using
calNIRSpecB, as described below. The pipeline will use the calibration
files and tables extracted from the Project Reference Database
Subsystem at STScI.
5.4
A Draft Calibration Pipeline “calNIRSpec”
The NIRSpec IDT has provided a draft structure for a spectroscopic pipeline (Boeker and
deMarchi memo of 15 June 2005 – see Figure 4) and there may be additional concepts or
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refinements under consideration. The following “straw-man” pipeline – meant only as a starting
point for discussions – is based upon initial understanding of the Boeker / deMarchi structure.
Do we also need an imaging pipeline? HST experience is that even target acquisition images get
used for science and should be calibrated – here we suggest an imaging pipeline (at least for
initial calibrations) that is essentially a subset of the processing flow for spectroscopy. After the
flux calibration step of the A portion, additional spectroscopic processing would occur that
would lead to additional spectroscopic data products. The part B (associated data) processing
flow will be somewhat different for imaging and spectroscopy. Part A applies to both imaging
and spectroscopy followed by a part B-imaging, or a part B-spectroscopy. Part B processing
deals primarily with pipeline processing of sub-pixel scale dithered data, which remains an
outstanding question to be addressed by future STScI/IDT working groups.
The steps necessary to implement Level 2 and Level 3 processing for NIRSpec images can be
illustrated in the context of draft calibration pipelines, which are tentatively designated
calNIRSpecA and calNIRSpecB, respectively. The two routines perform different
operations:
1. calNIRSpecA: This routine operates on the raw science data files and
removes the
Instrumental signature from the science data. It is the first calibration step,
and is applied to the data from each individual SCA that follows each
individual detector reset.
2. calNIRSpecB: This routine operates on associations: it co-adds datasets
obtained from multiple iterations of the same exposure and mosaics images
obtained from dither patterns.
Due to the different processing sequences to be suggested for imaging and spectroscopic modes,
we initially suggest the designations “-imag” and “-spec” might be added to the calNIRSpecB
nomenclature resulting in separate pipeline processing sequence designations for these modes.
These processing sequences are outlined in the next sections in terms of tables that represent
draft flow charts. The present drafts utilize the NIRSpec IDT draft spectroscopic pipeline
concept depicted in Figure 4 (T. Boeker and G. deMarchi – memo 15 June 2005) and some
detailed framework drawn heavily from the approaches considered by other JWST SIs,
particularly NIRCam (STScI-JWST-TM-2004-0022 by P. McCullough et al.), MIRI (STScIJWST-TM-2005-nnnn by S. Friedman et al. and JPL-D-25634 by J. Rhoads & M. Meixner), and
FGS-TFI (STScI-JWST-2005-00625 by A. Fullerton). These in turn enjoy heritage with the
calibration of HST instruments. However, other architectures are possible, and may be
preferable. The final design philosophy, ordering, and algorithms that constitute Level 2 and
Level 3 processing will depend in part on information about the performance characteristics of
the NIRSpec derived during AIT and upon detailed calibration working group review.
5.4.1 Level 2 Processing: (calNIRSpecA)
calNIRSpecA is the pipeline processing system to remove signatures imposed on NIRSpec
images by the blocking-filters / disperser combinations, fore optics, cameras, and detectors. It
applies to observations made in both imaging and spectroscopic mode. The processing is
performed individually for each MULTIACCUM exposure and SCA. A separate array of
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uncertainties records the result of the errors propagated through each step. Sub array data will be
processed through calNIRSpecA in the same way as full-frame readouts, with two exceptions:
a) Since reference pixels may not be included in the sub array, it may not be possible to use
them to correct for voltage drifts. This is not likely to be a problem, since sub array
exposures are reserved for very bright sources, which implies that the exposure times will
be short and any voltage drifts will be small.
b) The full distortion correction may not be required.
Table 4 represents an initial flow chart for calNIRSpecA. It also indicates what auxiliary
information – either in the form of reference files, or information computed from the data itself –
is required to perform each step (a full discussion of the steps in the calNIRSpecA and, to a
lesser extent, calNIRSpecB tasks is provided in Appendix A). For each individual exposure, the
result of calNIRSpecA is
a) For imaging:
a. A geometrically and photometrically corrected image taken through a filter whose
approximate central wavelength is known for each pixel in the field of view; and
b. An error array containing a realistic measure of the uncertainty in the flux of each
pixel.
b) For spectroscopy:
a. A geometrically corrected image modified by a “throughput correction” which
includes L-flat, blaze function, transmission of all optics, and a “default”
chromatic slit loss (that assumes perfect slit centering); and
b. An error array containing a realistic measure of the uncertainty in the corrected
count-rate of each pixel.
c. Photometrically corrected, extracted spectra for each for the fixed slits or
designated MSA slit locations.
d. For the IFU, a photometrically corrected data cube.
The issue of dithering exposures and how to combine them may lead to modifications of this
methodology. Alternate approaches to dithering are being considered at present. Will a
different dithering approach require modifications to part B reduction structure?
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Figure 4: Draft outline of NIRSpec pipeline
Step
Description
Reference File / Other Information
1.
For data from all modes, subtract the signal that
accumulates between the initializing reset and the
first sample in the MULTIACCUM sequence.
A reference file containing the “super Zero” read
frame containing the mean of many “first reads”
taken without illumination.
2.
Subtract the first read of the
MULTIACCUM sequence to remove
the reset (“ktc”) noise.
3.
4.
5.
Mask bad pixels to prevent them from
influencing subsequent processing steps. “Bad”
pixels may have uncharacteristic sensitivities
(“hot” or “cold”) or may be completely
inoperable.
Calibrate and remove drifts in the voltage by
manipulating the information from the reference
pixels and subtracting it from the array.
Remove the signal attributable to “dark current”
(and scattered/stray light) by subtracting a dark
exposure with the same integration time and
MULTIACCUM pattern as the science exposure.
A reference file containing a bad pixel mask.
A reference file containing a library of dark
current images.
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6.
7.
8.
9
Description
Correct for nonlinearity in the detector response
and flag saturated pixels.
Determine the count rate by measuring the rate of
accumulation of charge; i.e., the slope associated
with the “up-the-ramp” sampling. Outlying data
points, which are attributable to cosmic rays, are
flagged and eliminated from subsequent iterations
to determine the optimal slope. This step
proceeds pixel-by-pixel.
Correct for pixel-to-pixel sensitivity variations by
dividing by a “Pflat”. This might also serve as an
Lflat to remove some lower-spatial frequency
variations (e.g., fringing).
Imaging: Correct for image distortion.
Depending on the application, for imaging mode
this step might be more appropriate for Level 3
processing.
Spectroscopy: extract set of pixels illuminated by
each aperture or shutter; subsequent operations
are performed on each extraction box separately.
Imaging: Apply the photometric calibration.
(TBD: The appropriate calibration information
may be written into the header rather than
altering the data values themselves.)
10.
11.
12.
Spectroscopy: Apply the combined
“throughput corrections” including Lflat correction. (Data are modified)
For imaging and for spectroscopy with
MSA and non-IFU fixed apertures:
Output is 2-d image and 2-d
uncertainties;
For IFU: Output is count rate and
uncertainty data cubes
=============================
From this point on additional processing
applies ONLY to SPECTROSCOPIC
modes
Apply wavelength calibration and
perform geometric distortion correction;
will probably have to use model-based
correction for MOS; (some differences
for IFU?)
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Reference File / Other Information
A reference file of images containing
coefficients of the polynomial
correction factor.
A reference file containing a library of Pflats for
each filter. Alternately, a contemporaneous
Pflat.
A reference file containing the polynomial
coefficients required to remove instrumental
distortion from the images; for spectroscopy this
will include MSA configuration. There are
trade-offs in the implementation of this
correction, depending on whether flux should be
conserved for point sources or surface
brightness.
For imaging: a reference file
containing a library of coefficients to
transform instrumental fluxes to
calibrated fluxes, one for each filter.
Spectroscopy: a reference file that
includes L-flat, blaze function, optics
transmission, “default” chromatic slit
loss)
Requires reference files for grating equation,
(grating wheel telemetry?), and geometric
distortion map - corrects for tilt of aperture
relative to detector columns; also correct for tilt
of spectrum as function of wavelength and
disperser; dispersion model for MOS.
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Step
13.
14.
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Description
Apply absolute flux calibration.
Output is data cube for IFU and 2-d
image for MSA and fixed apertures.
Extract 1-d spectrum as appropriate
Subtract background
15.
16.
17.
Output 1-d extracted spectrum file for
each MSA aperture or fixed aperture.
Should there be any re-sampled
extracted products output for IFU?
OPTIONAL: apply “delta” chromatic
slit loss correction and produce updated
1-d extracted spectra
Reference File / Other Information
Requires flux calibration (sensitivity or
response function) reference file.
Requires background model or uses
nearby background determined from
unilluminated portion of detector. (Are
other options being considered?)
Structure is TBD – separate imsets of
fluxes and uncertainties for each slit
window or separate files?
Requires model or measurement-based reference
file or algorithm; current suggestion is to not
include in pipeline and perform this correction in
post-processing with a user-interactive tool.
Table 5 : Draft Flow Chart for calNIRSpecA
5.4.2 Level 3 Processing: (calNIRSpecB)
Currently we include separate imaging and spectroscopy branches (this could be handled as
internal branches within the part B code). IFU processing inputs and procedures may differ
somewhat from those for MSA and fixed slits.
calNIRSpecB is the pipeline processing system to combine associated images of a particular data
set. These families of images will generally arise from dithered pointings.
For imagery, the algorithms required to implement these steps generally require catalogs of
sources, and the generation of such lists is a necessary ingredient for Level 3 processing. An
enhanced version of this goal would generate source catalogs that are also useful for scientific
research.
For spectroscopy, information about target spacing is useful in registration of dithered images
that will lead to determination of target positioning within the fixed or pseudo apertures. This
information will be necessary for the determination of chromatic slit-loss corrections.
However, in any event, it is clear that the data products produced by calNIRSpecB are likely to
be the primary resource for subsequent analysis by astronomers.
There are a variety of approaches to achieving the goals of Level 3 processing. Tables 5 and 6
sketch one possible path. Since the concepts and algorithms required to implement
calNIRSpecB are likely to evolve substantially, it is necessarily less detailed than calNIRSpecA
at this stage. However, it is important to note that unlike calNIRSpecA, calNIRSpecB does not
require additional reference file or calibration information apart from PSF information necessary
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for applying chromatic slit-loss corrections. The information required to drizzle associated
images together or reconstruct data cubes resides in the files that are input.
Since Level 3 processing of data will likely require a certain amount of tailoring that in turn
depends on the scientific goal of the observation, it may be preferable to leave it up to the
investigators themselves rather than a general pipeline process operated by the S&OC. This is
especially true if no optimal method for determining slit-corrections can be automated.
Additional post-pipeline processing tools:
•
•
•
•
PSF subtraction, in order to obtain enhanced dynamic range in the final image. The PSF
will be determined from contemporaneous observations of a suitable target with the same
instrumental configuration that was used for the science target.
Coaddition of PSF-subtracted frames obtained at multiple roll angles.
LSF characterizations and tools to be used to enhance resolution of blended features
Corrections for extended objects
Step Description
1.
Remarks
Start with photometrically-corrected output
image for each dithered image from NIRSpecA
Identify sources in all associated images.
2.
3.
4.
5.
Use the source lists to refine the astrometric
transformations between images.
Use object-free regions to balance the
background in different images
Combine the dithered images into a mosaic.
6.
Output is 2-d image and 2-d
uncertainties
7.
OPTION: Extract a scientifically meaningful
source catalog from an observation.
The SExtractor software package (Bertin &
Arnouts 1996, A&AS, 117, 393) provides an
example of a flexible, efficient source-detection
algorithm that could be used.
The result is the highest quality image of a field.
Table 6: Draft Flow Chart for Level 3 calNIRSpecB-imag Processing
Step Description
0.
Remarks
If no correction is applied for slit mis-centering
or chromatic slit-loss then errors associated with
combination of dithered or multiple images are a
lower limit. Will a NIRSpecB combination be
performed in that case?
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1.
2.
3.
4.
MSA and Fixed Slits: Start with throughput
corrected output image for each dithered image
from NIRSpecA
IFU: Start with corrected count-rate data cube?
Identify spectra in all associated images.
Need input from ongoing discussions between
STScI and IDT on status of suggestions for
combining dithered images.
Use the source lists to refine the astrometric
transformations between images.
Use object-free regions to balance the
background in different images
Combine the dithered images into a mosaic.
5.
6.
7.
Output is 2-d image and 2-d uncertainties or (for
IFU) a data cube
Perform steps 12-16 of NIRSpecA on mosaic
image. If process has optimally identified and
corrected for slit losses, then step 17 of
NIRSpecA is not required.
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Calibration working group discussion needed
The result is claimed to allow the determination
of the object centering in the aperture for each
separate image and thereby, to allow aperture
corrections to be applied in a pipeline fashion.
Will this work as well for extended sources?
What is correct order of operations?
This depends on slit-loss correction
algorithm, and possibly, on aperture
mode (IFU).
Table 7: Draft Flow Chart for Level 3 calNIRSpecB-spec Processing
5.4.3
IFU Level 2 and 3 Processing
More detail is needed concerning the production of the data cube in Level 2 processing and then
for Level 3 processing through extraction of 1-d spectra.
The data reduction procedures for IFU spectroscopy consist of all of the routines that are
applicable (separately) to imaging and spectroscopy as well as those that define how the final 3D data cube is created (and calibrated). IFU calibration will be very similar to other processing
for initial portions of calNIRSpecA, but there will likely be important differences from other
modes (arising from the complex mapping from pixel location and grating setting to celestial
coordinates and wavelength for the data cube) at the flat fielding, wavelength calibration, and
spectral extraction stages. Some implications for flat fielding, wavelength calibration, and
spectral extraction are discussed in the following subsections:
For Level 3 IFU data processing the initial step, combining data from multiple Level 2 processed
dither positions, differs from the corresponding steps in imaging data because the each of the
input Level 2 processed IFU observations are, in principle, a data cube (should the presumably
geometrically corrected input cube be in count-rate units or flux-calibrated?). A threedimensional analog of the drizzle algorithm or similar method may be required for full
generality, though it may also be possible to treat the problem as a set of 2D mosaics of
monochromatic images provided the wavelength grid is sufficiently stable among the different
exposures being stacked. This simplification would be possible when a single grating position
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has been used, but might be problematic when splicing together data taken in multiple grating
settings.
5.4.3.1 IFU Spectrum Extraction
In principle the spectrum can be extracted for each position in the spatial dimension. Is this
desirable? Should this be performed in the pipeline or in user-specific post-processing? At
present there have been no discussions or presentations on the details of IFU spectrum
extraction. This is likely to be an important calibration working group item. Full exploitation of
IFU data also may require measuring the point spread and line-spread functions for this mode.
These measurements can likely be extracted from astrometric calibration data for the PSF, and
wavelength calibration data for the LSF.
5.4.3.2 IFU Astrometric and Wavelength Calibration
The location of the spatial and spectral information on the FPA for IFU mode will be established
along with other modes in ground testing. Monochromatic light filling the field would provide
an ideal test of wavelength zero point and dispersion, while continuum point source observations
at a grid of locations will provide an ideal measurement of the mapping between pixels and
celestial coordinates.
We will need to verify these locations in orbit and perhaps monitor them because the integrity of
the data cube depends on it. A celestial spectral source, e.g. a compact planetary nebula, can be
stepped across the IFU and for each position of the source a data cube be made. The exact
position of the source in the data cube in both spatial and spectral dimensions will be checked
throughout the science mission.
The mapping from pixel coordinates to celestial coordinates and wavelength can be stored as a
set of three real-valued images, corresponding to offsets relative to the data cube center (α, δ, λ)
in two orthogonal sky coordinates and wavelength. Each separate grating setting would require
its own wavelength mapping (x,y  λ). Ground testing will show whether a single (x,y  α,δ)
mapping is sufficient for all grating positions or if the mapping also needs to be derived for each
grating position separately. The effective volume corresponding to each detector pixel can be
obtained as a numerical derivative of these maps.
5.4.3.3 IFU Flat Field Calibration
Wavelength-dependent flat fields may be needed for IFU calibration. The extent of wavelength
dependence and the need for flat fields with each grating position must be determined. Flat
fields for the IFU mode may be obtained by using the internal calibration unit or by using
celestial sources. External flats could be obtained using two types of celestial sources. First,
spectrophotometric standard stars may be observed at a few locations in the field of view to
determine the response as a function of wavelength in absolute units. If moving target capability
is developed, then a bright standard could effectively be trailed through the IFU slit.
Alternatively, the results could be transferred to the entire field of view by observing extended
objects of uniform (or well known) surface brightness profile. For example, the MIRI calibration
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plan suggests that the outer planets Uranus and Neptune are promising extended sources for
generating an IFU flat field, and this may be a possibility for NIRSpec as well. In addition to
handling variations in quantum efficiency, the flat fielding stage of IFU calibrations may have to
incorporate fringing corrections. Fringing will be characterized in ground testing. This
characterization will be confirmed during on-orbit checkout of the instrument.
5.5 Baseline Strategy for Supplying and Maintaining Reference Files
The draft version of the calNIRSpecA pipeline outlined in Table 4 requires at least 11 types of
reference files to inform the processing algorithms of the specific properties for the various
modes of NIRSpec. Strategies for obtaining the information necessary to generate these files at
various stages in the life of the instrument have been briefly discussed in previous sections of
this document. This information is summarized in Table 7, along with an initial indication of the
frequency that will be required to monitor the on-orbit behavior of the properties of the NIRSpec
that are contained in the reference files.
An important lesson learned from the development and operation of first- and second-generation
HST instruments and which has been implemented as routine procedure for COS and WFC3 is
that it is important to establish a philosophy early in instrument development of archiving in
MAST all data that can lead to reference file production. A very valuable procedure is to
establish an agreement that when reference files are produced (regardless of software used) that
the starting dataset ALWAYS be drawn from the MAST archive.
Reference File
“Super Zero”
Frame
Bad Pixel Map
Library of
Darks
AIT
Average many
“first reads”
from dark
images obtained
from library.
Obtain from
measurements of
average Pflat;
also from dark
images.
Obtain during
TVAC testing
with standard
MULTIACCUM
sequences.
Commissioning
Routine
Observing
Monitor
performance in
calibration of
science images.
On-orbit
Frequency
~Weekly (?)
Obtain from
“parallel”
darks.
Verify groundbased values from
observations of
darks and Pflats.
Monitor
Obtain with
standard
MULTIACCUM
sequences at
standard operating
temperature.
Monitor
performance in
calibration of
science images.
~Weekly;
Use “parallel”
darks to search
for new hot
pixels.
~Weekly (?)
Obtained as
parallel
observation.
Average many
“first reads” from
dark images.
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Nonlinearity
Correction
Library of Pflats
Photometric
Calibration
Wavelength
Obtain initial
(relative)
estimate from
pixel-pixel
observations
sources of
known
brightness
During TVAC
testing.
Observations of
primary and
secondary
standards covering
a large range of
fluxes; e.g., as part
of photometric
calibration; but
only for a small
grid of pixels,
since relative
behavior
established.
Obtain during
Obtain with
TVAC tests with observations of
GSE or internal
internal source for
lamp, for
a large subset of
many/all
blocking
filter/disperser
filter/disperser
combinations.
combinations.
Only coarse
Establish baseline
absolute
calibration by
calibration
observing external
achievable but
standards at a
effort should be
variety of
made to establish filter/disperser
relative
combinations.
throughputs for
all
filter/disperser
combinations.
Establish by
Confirm with
using
internal
wavelength
wavelength
calibration
calibration source;
source and
also with
instrumental
astronomical
models.
objects with many
bright emission
lines.
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Verify quality of
correction as part
of routine
photometric
monitoring.
Infrequent;
Possibly only
once unless
monitoring
suggests more
data required.
?
~Monthly(?).
Monitor available
primary/secondary
standards in
routine calibration
plan observations.
.
~Once or twice
per year on
orbit?
Perform
contamination
monitor more
frequently
(monthly?)
Prior or after each
new
filter/disperser
combination
observing
sequence? .
Monthly;
Weekly if
concerned
about
mechanism
repeatability?
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Overview of NIRSpec Calibration Activities
“Transmission
Correction”
(Combined with
L-flat)
Distortion
Correction
Measure relative
throughputs as
function of
wavelength for
each filter, optic,
aperture,
Estimate from
models; direct
measurement
only possible in
AIT.
Ray tracing;
Pinhole maps
(?). At ambient
or during TVAC
with GSE.
Astrometric
Solutions File
PSF model file
LSF files
Reference Pixel
Info
Slit locations,
IFU slice
positions
Obtain baseline
optimum PSFs
from alignment
activities with
FM.
Obtain baseline
LSFs from
wavecal
activities with
FM.
Obtain positions
from routine
calibration darks
to establish
initial
characterization.
Establish with
special
calibration
program with
FM.
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Establish baseline
calibration by
observing external
standards at a
variety of
filter/disperser
combinations and
combine with
detailed
instrumental
modeling.
Deep imaging of
an astrometric
field at several roll
angles.
External
observations
conducted as part
of photometric
calibration.
~Once or twice
per year on
orbit?
Verify efficacy of
correction using
science images.
Annually.
Observe
astrometric field to
establish baseline
calibration.
Repeat
astrometric
calibration in
routine calibration
plan observations.
Repeat
serendipitous
observations in
routine calibration
plan
Semi-annual
checks?
Repeat
serendipitous
observations in
routine calibration
plan
Semi-annual
checks? Does
re-phasing
affect?
Monitor positions
from routine
calibration darks.
Weekly? Check
in each dark
set.
Repeat special
calibration in
routine calibration
plan observations.
Semi-annual
checks?
Extract PSF
information from
astrometric fields
and other
serendipitous
calibration images.
Utilize
serendipitous
internal and
external
wavelength
calibration data.
Monitor positions
from routine
calibration darks.
Establish baseline
with special
calibration
program.
Semi-annual
checks? Does
re-phasing
affect?
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Spectrum trace
orientation
correction
Establish in special
calibration program
with FM
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Establish baseline
with special
wavelength
calibration
program that
includes robust
line sources
Serendipitous
observations in
routine calibration
plan
Annually.
Table 6: Acquisition and Maintenance of Possible Reference Files
5.6
5.6.1
Summary of Required Calibration Reference Files by Mode
Fixed Slit and MSA Spectroscopy
Bad pixel mask
Dark frames
Short Dark frames (for Read noise measurements)
Reference pixels frames
Reference output frames
Internal flat fields for high frequency variations (P-flats)
Combined L-flat / throughput measures for low frequency variations
Saturation counts map
Linearity counts map
Geometric distortion map
Standard star measurements
Astrometric solutions file
Wavelength calibration file; grating parameters; dispersion model
Location of the slit information on the FPA
Line spread function information (for subsequent analysis).
5.6.2 IFU Spectroscopy
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
Bad pixel mask
Dark frames
Short Dark frames (for Read noise measurements)
Reference pixels frames
Reference output frames
Internal flat fields for high frequency variations (P-flats)
Combined L-flat / throughput measures for low frequency variations
Saturation counts map
Linearity counts map
Geometric distortion map
Standard star measurements
Astrometric solutions file
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13.
14.
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Table of slice and wavelength positions on the FPAs
Wavelength calibration spectra; grating parameters
5.6.3 IFU Photometric Imaging
1.
Bad pixel mask
2.
Library dark frames (where necessary - exposure time and readout sequence
matched to science exposures)
3.
Reference pixels (from each frame)
4.
Reference output line data (from each frame)
5.
Internal flat fields for high frequency variations (P-flats)
6.
Combined L-flat / throughput measures for low frequency variations?
7.
Saturation counts map
8.
Linearity counts map
9.
Geometric distortion map
10.
Standard star measurements
11.
Astrometric solutions file
12.
PSF model file (for subsequent analysis).
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Appendix A
Acronym/
Abbreviation
AD
ADU
AFP
AIT
BOL
BRDF
BTDF
C&DH or CDH
CDR
CRAL
CVZ
DHAS
DILS
DRM
DRD
EGSE
Eml
EMC
EMI
EOL
ESA
ESTEC
ETU
FF
FGS
FGS–G
FGS–TFI
FITS
FM
FOV
FPA
FPAP
FPE
FPM
FPS
FRD
FWHM
GO
GSE
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Acronym/Abbreviation List
Definition
Applicable Document
Analog-to-Digital Unit
Aperture Focal Plane
Assembly, Integration, & Test
Beginning Of Life
Bi-directional reflectance distribution function
Bi-directional transmittance distribution function
Command and Data Handling (Hardware)
Critical Design Review
Centre de Recherche Astronomique de Lyon
Continuous Viewing Zone
Data Handling and Analysis System
Deliverable Item List
Design Reference Mission
DRD Data Requirements Document
Electrical Ground Support Equipment
Engineering Model
Electromagnetic Compatibility
Electromagnetic Interference
End Of Life
European Space Agency
European Space Research and Space Technology Centre
Engineering Test Unit
Flat Field
Fine Guidance Sensor (complete instrument)
Fine Guidance Sensor – Guider (hardware and functions)
Fine Guidance Sensor – Tunable Filter Imager (hardware and functions)
Flexible Image Transport System
Flight Model
Field of View
Focal Plane Assembly (or Array)
Focal Plane Array Processor
Focal Plane Electronics (Science Instruments)
Focal Plane Module
Focal Plane System
Functional Requirements Document
Full Width at Half Maximum
General Observer
Ground Support Equipment
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Acronym/
Abbreviation
GSFC
HgCdTe
HST
I&T
I/O
ICD
IC&DH or
ICDH
ICE
IDT
IFU
IPS
IRD
ISIM
Jy
JWST
L2
LAM
LED
LMATC
LMC
LSF
L2
mas
MAST
MCE
MGSE
MIRI
MSA
NASA
NGST
NICMOS
NIRCam
NIRSpec
OA
OB or OBA
OGSE
OTE
PDR
PDU
POM
PRD
PSF
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Definition
Goddard Space Flight Center
Mercury Cadmium Telluride
Hubble Space Telescope
Integration & Test
Input-Output
Interface Control Document
ISIM Command & Data Handling
Instrument Control Electronics (Science Instruments)
Instrument Development Team
Integral Field Unit
NIRSpec Instrument Performance Simulator
Interface Requirement Document
Integrated Science Instrument Module
Jansky (a unit of flux; 1 Jy = 10-26 W m-2 Hz-1)
James Webb Space Telescope
Second Lagrange point of the earth – sun system
Laboratoire d’Astrophysique de Marseille
Light-Emitting Diode
Lockheed Martin Advanced Technology Center
Large Magellanic Cloud
Line Spread Function
L2 Second Lagrange Point
milli-arcseconds
Multi-Mission Archive at Space Telescope
Micro-shutter Control Electronics
Mechanical Ground Support Equipment
Mid-Infrared Instrument (JWST)
Micro-Shutter Array
National Aeronautics & Space Administration
Northrop Grumman Space & Technology
Near Infrared Camera and Multi-Object Spectrometer (HST)
Near-Infrared Camera (JWST)
Near-Infrared Spectrograph (JWST)
Optical Assembly
Optical Bench or Optical Bench Assembly
Optical Ground Support Equipment
Optical Telescope Element
Preliminary Design Review
Primary Data Unit (of a FITS file)
Pick-Off Mirror
Project Reference Database
Point Spread Function
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Overview of NIRSpec Calibration Activities
Acronym/
Abbreviation
R
RD
RMS or rms
ROE
RSS
S/C
S/N
S/W or SW
S&OC
SAM
SCA
SI
SIDU
SIRTF
SITS
SNR
SRA
SSR
STScI
TA
TBC
TBD
TBR
TVAC
V1
WCS
WFE
WFS
WFS&C
µm
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Definition
Resolution
Reference Document
Root Mean Square
Readout Electronics
Root Sum Squared
Spacecraft
Signal-to-Noise Ratio
Software
Science & Operations Center
Small Angle Maneuver
Sensor Chip Array (or Assembly)
Science Instrument (refers to JWST)
Science Instrument Development Unit
Space Infrared Telescope Facility
Science Instrument Test Set
Signal to Noise Ratio
Science Rationale & Analysis (document)
Solid State Recorder
Space Telescope Science Institute
Target Acquisition
To Be Confirmed
To Be Determined (or Defined)
To Be Reviewed (or Required, Revised, or Resolved)
Thermal Vacuum
Axis of the S/C - OTE - ISIM coordinate system; positive toward secondary
mirror
World Coordinate System
Wave Front Error
Wave-front Sensing
Wave-front Sensing & Control
Micrometers (microns)
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Appendix B
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References
Anderson, J. & King, I.R., 2000, PASP, 112, 1360
Anderson, J. & King, I.R., 2003, PASP, 115, 113
Bertin, E., & Arnouts, S., 1996, “SExtractor: Software for Source Extraction” A&A Supplements
117, 393.
Bohlin, R.C., 2000, AJ, 120, 437
Bohlin, R.C., Dickinson, M.E., & Calzetti, D., 2001, AJ, 122, 2118
Campins, H., Rieke, G.H., & Lebofsky, M.J., 1985, AJ, 90, 896
Fruchter, A. S., & Hook, R., 2002, “Drizzle: A Method for the Linear Reconstruction of
Undersampled Images,” PASP 114, 144.
Fullerton, A., 2005, “Overview of FGS-TF Imager Calibration Activities: STScI Input to CSA
Document OPS-03B,” STScI-JWST-2005-00625 (15 June 2005 Issue A)
Kriss, G.A., 2004, STScI-JWST-TM-2004-0025
McCullough, P. et al., 2004, STScI-JWST-TM-2004-0022
Monelli, M. et al., 2005, ApJ, 621, L117
Rauscher, B.J. et al. 2003, “Ultra-Low Background Operation of Near-Infrared Detectors Using
Reference Pixels for NGST”, Proceedings of SPIE
Regan, M. & Stockman, H. 2001, STScI-JWST-TM-2001Rhoads, J. & Meixner, M., 2005, JPL-D-25634
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Overview of NIRSpec Calibration Activities
Appendix C
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Draft NIRSpec On-Orbit Calibration Program Summaries
Program No.: 650
Program title: NIRSpec Dark Monitor
Synopsis:
This program will monitor stability of the dark rate in every NIRSpec detector pixel. One long dark
exposure will be obtained each week. The exposure length for the dark will be the maximum length
expected for science exposures, which is currently 10,000 seconds (TBR). Empirical darks for shorter
exposures can be extracted from the raw data for the long exposure by ignoring later reads. Model darks
for any exposure time can be determined by fitting the sequence of reads in the long exposure. If
commissioning uncovers variations in the dark rate on timescales shorter than a week, shorter timescales,
more frequent darks may be required. Darks from multiple epochs will likely be combined automatically
to construct “super-darks” with lower noise. There is no detailed calibration plan for NIRSpec yet, so
details of this program are preliminary.
Sample and Sky Coverage:
N/A
Basis for exposure time estimates (S/N & brightness):
Instruments and observing configurations:
NIRSpec, dark, both detectors, 10000 seconds
Scheduling requirements or constraints:
Darks should be scheduled within windows 4 days wide, centered every 7 days. If operationally
supported, darks should be obtained in parallel with normal science observations.
Visit scenarios:
One long dark exposure, in parallel if supported
Total program time:
6 days
Program written by:
Jeff Valenti
Date first written:
12/07/2004
As-of date:
2004 December 07
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Program No.: 651
Program title: NIRSpec Flat Field Monitor
Synopsis:
This program will monitor stability of the NIRSpec flat-field response. Flats will be obtained every 4
months, using the three R=1000 gratings with three different long-slit configurations of the micro-shutter
array (MSA), for a total of nine exposures per epoch. Use of gratings, rather than the imaging mirror,
provides information about flat-field response at 3 to 9 distinct wavelengths per pixel and avoids smallscale structure along the dispersion axis caused by the MSA frame. The MSA will still cause small-scale
structure along the spatial axis. The long slits will be located near both edges of the MSA (along the
dispersion axis) and near the middle. The exact locations of long slits will be chosen to minimize gaps
due to permanently closed shutters. The long slits may be more than one shutter wide, if necessary to
achieve the desired level of illumination and to ensure at least some illumination, even if a few shutters
are permanently closed. There is no detailed calibration plan for NIRSpec yet, so details of this program
are preliminary.
Sample and Sky Coverage:
Internal lamp.
Basis for exposure time estimates (S/N & brightness):
Instruments and observing configurations:
Design of the calibration system is not yet mature enough to estimate exposure times. We provide
maximum desired exposure times here, rather than predicted exposure times. Actual exposure times will
only be comparable for all three gratings if the lamp has selectable brightness.
NIRSpec, G140M, 3x1800s (TBR)
NIRSpec, G235M, 3x1800s (TBR)
NIRSpec, G395M, 3x1800s (TBR)
Scheduling requirements or constraints:
All 9 exposures should be scheduled within windows 1 month wide, centered every 4 months. If
operationally supported, flats should be obtained during slews or in parallel with science observations, if
the NIRSpec internal lamp does not interfere with the science observation.
Visit scenarios:
Visit lengths should be no longer than 1800s (TBR)
Total program time:
0.5 day
Program written by:
Jeff Valenti
Date first written:
12/08/2004
As-of date:
2004 December 07
2
Check with the JWST S&OC OCM to verify that this is the correct version prior to use.
Overview of NIRSpec Calibration Activities
JWST-STScI-000851, SM-12
-
Program No.: 652
Program title: NIRSpec MSA to Detector Distortion Monitor
Synopsis:
This program will monitor NIRSpec geometric distortion between the micro-shutter array (MSA) and the
two detectors, using in internal lamp source and regular patterns of open and closed MSA shutters.
Measurements will be obtained every 3 months, within a day of the external observations used to calibrate
the distortion between the sky and the NIRSpec MSA. Frequent monitoring is warranted because good
instrumental sensitivity relies on accurate positioning in the apertures, which in turn relies on an accurate
distortion map. There is no detailed calibration plan for NIRSpec yet, so details of this program are
preliminary.
Sample and Sky Coverage:
Internal lamp source
Basis for exposure time estimates (S/N & brightness):
Instruments and observing configurations:
NIRSpec, Mirror, 6x12s (TBR)
Scheduling requirements or constraints:
Each visit should be scheduled within 1 day of observations for program 661 (NIRSpec Sky to MSA
Distortion Monitor).
Visit scenarios:
At each epoch, images will be obtained with three MSA configurations designed to map geometric
distortion along the dispersion direction and three configurations designed to map distortion along the
spatial axis.
Total program time:
0.1 day
Program written by:
Jeff Valenti
Date first written:
12/08/2004
As-of date:
2004 December 08
3
Check with the JWST S&OC OCM to verify that this is the correct version prior to use.
Overview of NIRSpec Calibration Activities
JWST-STScI-000851, SM-12
-
Program No.: 653
Program title: NIRSpec Wavelength Monitor
Synopsis:
This program will monitor the mapping between detector pixels and wavelength of dispersed light, using
in deep exposures of an internal line lamp and regular patterns of open and closed MSA shutters. Five
different MSA configurations will be used to measure how the wavelength mapping varies with position
of the shutter across the MSA. Multiple shutters may be opened at each spatial position on the MSA, if
the line density is low enough that overlapping spectra can be disentangled. Overlap of spectra will occur
anyway at spatial positions where a shutter has failed open. Measurements will be obtained every 3
months. There is no detailed calibration plan for NIRSpec yet, so details of this program are preliminary.
Sample and Sky Coverage:
Internal wavelength calibration source
Basis for exposure time estimates (S/N & brightness):
Instruments and observing configurations:
The following exposure times are based on the speculation that exposure times will be no longer than 50
seconds for wavelength calibration spectra obtained contemporaneously with science observations, and
that the deep wavelength calibration exposures will require exposure times 5 times longer.
NIRSpec, P285L, 5x300s (TBR)
NIRSpec, G140M, 5x300s (TBR)
NIRSpec, G235M, 5x300s (TBR)
NIRSpec, G395M, 5x300s (TBR)
NIRSpec, G140H, 5x300s (TBR)
NIRSpec, G235M, 5x300s (TBR)
NIRSpec, G395H, 5x300s (TBR)
Scheduling requirements or constraints:
All observations for a given disperser must be obtained in the same visit, so that the grating does not
move between exposures. Observations with each grating may be placed in separate visits.
Visit scenarios:
Visits should be scheduled within windows 1 month wide, centered every 3 months. At each epoch,
images will be obtained with 5 MSA configurations, designed to determine how wavelength solution
depends on location of the shutter in the MSA.
Total program time needed:
0.25 day
Program written by:
Jeff Valenti
Date first written:
12/09/2004
As-of date:
2005 January 19
4
Check with the JWST S&OC OCM to verify that this is the correct version prior to use.
Overview of NIRSpec Calibration Activities
JWST-STScI-000851, SM-12
-
Program No.: 660
Program title: NIRSpec Spectrophotometric Throughput Monitor
Synopsis:
This program will monitor NIRSpec spectrophotometric throughput for all 7 dispersing elements.
Measurements will be obtained annually, using the 5 fixed slits, the integral field unit (IFU), and one
aperture per micro-shutter array (MSA) quadrant, for a total of 10 apertures. Not every combination of
dispersing element and aperture is allowed. Nine different sub-aperture positions will be used with the
prism to characterize spectrophotometric throughput as a function of location in each aperture. There is no
detailed calibration plan for NIRSpec yet, so details of this program are preliminary.
Sample and Sky Coverage:
The white dwarf G191B2B is the preferred primary spectrophotometric standard. With ecliptic latitude of
+30 degrees, G191B2B is accessible only part of the year. Possible alternate standards include Hz 43, GD
153, and GD 71. See Bohlin (2000, AJ, 120, 437).
Basis for exposure time estimates (S/N & brightness):
Instruments and observing configurations:
The exposure time estimates below come from the Exposure Time Calculator (ETC) in the JWST Mission
Simulator (JMS).
NIRSpec, P285L, 90x120s (TBR)
NIRSpec, G140M, 10x180s (TBR)
NIRSpec, G235M, 10x480s (TBR)
NIRSpec, G395M, 10x1200s (TBR)
NIRSpec, G140H, 10x480s (TBR)
NIRSpec, G235M, 10x1440s (TBR)
NIRSpec, G395H, 10x3600s (TBR)
Scheduling requirements or constraints:
To minimize overheads associated with slews, all exposures in a given year should be scheduled in a
single visit, if possible. Each annual set of exposures should be scheduled within windows 2 months wide,
centered every 12 months.
Visit scenarios:
The flux standard will be observed with each disperser in succession. In principle, observations with each
disperser can be split into different visits, but this will reduce efficiency. For each disperser, the flux
standard will be observed with each allowed aperture in succession. For the prism only, 9 different subaperture locations (arranged in a “+” pattern) will be used for each aperture. Subarrays may be used when
exposure times are too short to readout the entire detector.
Total program time:
1.0 day
Program written by:
Jeff Valenti
Date first written:
12/08/2004
As-of date:
2004 December 08
5
Check with the JWST S&OC OCM to verify that this is the correct version prior to use.
Overview of NIRSpec Calibration Activities
JWST-STScI-000851, SM-12
-
Program No.: 661
Program title: NIRSpec Sky to MSA Distortion Monitor
Synopsis:
This program will monitor NIRSpec geometric distortion between the sky and the micro-shutter array
(MSA), using an astrometrically calibrated external star field. Measurements will be obtained every 3
months. Frequent monitoring is warranted because good instrumental sensitivity relies on accurate
positioning in the apertures, which in turn relies on an accurate distortion map. At each epoch, dithered
images will be obtained at two spacecraft roll angles with the maximum possible separation to better
constrain the distortion model. There is no detailed calibration plan for NIRSpec yet, so details of this
program are preliminary.
Sample and sky coverage:
A field in the Large Magellenic Cloud (LMC) visible throughout the year will be selected and calibrated
astrometrically, using NIRCam and perhaps HST/ACS.
Basis for exposure time estimates (S/N & brightness):
Instruments and observing configurations:
NIRSpec, Mirror, 2x5x1800s (TBR)
Scheduling requirements or constraints:
Each visit should be scheduled within windows 1 month wide, centered every 3 months.
Visit scenarios:
At each epoch, images will be obtained at two maximally separated roll angles. At each roll angle, images
will be obtained at 5 (TBR) different dither positions with offsets as large as half the 3.3’ field of view.
Multiple guide stars (hence visits) will be necessary to support the large offsets. Visits for a given epoch
should be executed consecutively, without interruption.
Total program:
0.6 day
Program written by:
Jeff Valenti
Date first written:
12/08/2004
As-of date:
2004 December 08
6
Check with the JWST S&OC OCM to verify that this is the correct version prior to use.