TECHNICAL
REPORT
Title: Observing scenarios for NIRSpec:
Initial release
Doc #:
Date:
Rev:
Authors: D. Soderblom,
T. Beck, K. Gordon, D.
Karakla, T. Keyes, D.
Long, J. Muzerolle, J.
Tumlinson, and J. Valenti
Release Date: 24 February 2011
Phone: 410338-4543
JWST-STScI-002270, SM-12
October 6, 2010
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1.0 Abstract
This report summarizes test cases created to examine and verify the capabilities of the
JWST ground system for NIRSpec, particularly the planned NIRSpec templates. These
observing scenarios were created to represent a broad range of potential science programs
likely to be undertaken by NIRSpec users as a means of testing existing template
concepts and potential needs for user info0rmation and software. The lessons learned are
summarized, with recommendations for potential changes and additions to the templates.
2.0 Introduction
JWST is being designed and built with several key scientific goals in mind and it is also
intended to be a general-purpose observatory for infrared astronomical observations (see
JWST-RQMT-002558, James Webb Space Telescope Project, Science Requirements
Document, J. Mather). The telescope and its instruments have been specified and built to
be able to achieve those key scientific goals. Several years ago an effort was undertaken
to draft a Science Operations Design Reference Mission (see JWST-STScI-000373,
Science Operations Design Reference Mission (SODRM), Phase 1 Proposals, L. Petro).
The primary purpose of the SODRM was to examine realistic scientific uses of JWST
and to see how they could be accommodated within the design constraints then
considered and to estimate the overall observing efficiency of the observatory.
Since the SODRM was completed, the ground system for JWST has been specified and is
now being created and tested. The capabilities of JWST’s instruments are made available
to observers via “templates” in APT, the Astronomers’ Proposal Tool. These templates
combine instrument settings and constrain user selections; with the goal of simplifying
construction of Phase II programs and simplifying the software that supports that. These
templates can either support or inhibit the kinds of observations that can be performed
with the observatory, and those capabilities also affect significantly the efficiency of how
observations are carried out.
Operated by the Association of Universities for Research in Astronomy, Inc., for the National
Aeronautics and Space Administration under Contract NAS5-03127
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The Webb Instruments Team recently began an effort to understand better the capabilities
and limitations of the templates in order to verify that critical capabilities are present and
to help set priorities for any additional capabilities that may be required. This Technical
Report summarizes results for JWST’s Near-Infrared Spectrograph (NIRSpec).
The scenarios outlined here are not completely defined observing programs yet, in part
because of the effort needed to do that and in part because not enough detailed
information yet exists on the usage of JWST. A major goal of creating these scenarios is,
in fact, to help guide choices in making the observatory and its capabilities more fully
defined. This report is being issued in its current form to provide the basis for further
discussion and decisions, with the expectation of significant revisions as time goes on.
For example, these scenarios could form the basis for other user studies, such as how
observer-supplied information gets fed to the data management system, or how
documentation can better meet user needs.
3.0 Observing scenarios
In undertaking this study, the WIT NIRSpec group first discussed the general science
topics to be covered and the methods to be used in these studies, and individuals then
volunteered to work on specific scenarios. The primary aim is to ensure that necessary
capabilities are in the ground system to enable NIRSpec to meet its science goals. A
secondary goal is to identify areas where changes could lead to significant efficiency
improvements. It is also important to minimize the usage of the mechanisms on JWST,
particularly reconfigurations of NIRSpec’s Micro-Shutter Arrays (MSAs), and so the use
cases have been examined with a view to efficiency in a broad sense.
The procedure used included these steps:
•
Describe the science goals sufficiently to determine the observing parameters that
would be required, such as number of targets and grating-filter combination.
•
Create a candidate source list with flux estimates or ranges of fluxes.
•
Estimate exposure times needed, based on existing knowledge of sensitivity, and
select a dither pattern based on the science requirements.
•
Lay out the likely exposures in detail, including preferred usage of MSA shutters
for targets and backgrounds. The details of the MSAs (closed and open shutters)
were not considered at this stage.
•
Describe verbally the considerations used in creating the scenario as reference.
•
List the pros and cons of the existing template structures in building the
observations and what changes to the templates could lead to better observing
efficiency or better science capabilities.
For these scenarios we did not go as far as choosing guide stars because that seemed
unnecessary at this stage.
The observing scenarios that have been created are detailed in the Appendix. Here we
provide short summaries so as to provide a listing of what has been treated.
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Program Title
Author
Mode
Features
200
Kinematics of stars in the Galactic
Center
Valenti
MSA
crowded field;
multiple pointings
201
Evolution of ices in star-forming
environments
Beck
MSA
202
YSO jets near IRS-1 in NGC 2264
Karakla
MSA
extended objects;
bright sources
203
Massive star-forming regions in
the Milky Way
Muzerolle
MSA
very crowded region;
large brightness range
204
First-light galaxies in the Hubble
Ultra-Deep Field
Soderblom
MSA
very long integrations;
very faint sources
205
Carbon abundances in Omega
Centauri
Tumlinson
MSA
extremely crowded field
207
MSA spectroscopy of a very
extended object
Keyes
MSA
coverage efficiency
230
NIRSpec follow-up of Gamma-ray
burst afterglows
Tumlinson
Fixed
Slit
Quick turnaround TOO
231
Exoplanet atmospheres
Valenti
Fixed
Slit
high signal-to-noise;
critical timing
261
Atomic hydrogen filaments in
Perseus A (NGC 1275)
Beck
IFU
mosaic, large field
502
MIRI/NIRSpec IFU observations of
extragalactic H II regions
Gordon
IFU
multi-instrument
4.0 Lessons Learned
The scenarios described here were intentionally chosen to exercise the full range of
NIRSpec’s functional capabilities, and that means that as an ensemble they are probably
not typical of what will be executed with the instrument on-orbit. At the same time, the
range of modes used and the science topics covered means that these scenarios should
test the bounds of the ways in which general observers will use the instrument for
scientific purposes. Also, some of the observations described here are based on some of
the scientific motivations for building JWST in the first place, and the capabilities needed
to execute them are among the highest-level requirements on the observatory.
The scenarios were constructed with the goal of identifying obstructions or observational
inefficiencies in the systems that observers will use. Some inefficiencies result when
more overall time is required to execute a series of observations than would be the case
with a modest change to the software.. Other inefficiencies lead to more reconfigurations
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of the MSA – a limited lifetime item – than would be the case if other options were
available.
4.1
Templates:
Each of the scenarios described includes concerns identified with the current templates.
They are listed here for conciseness. In many cases each concern identified applies to
several of the scenarios.
• Single visits need to be able to carry out multiple target acquisitions. This need arises
if the total visit length exceeds ~10,000 sec or if a dither of more than ~5 arcsec is
needed. Both situations are likely to be common.
• A single visit may now include multiple target sets, which aids efficiency
significantly. However, current capabilities only allow for a single acquisition
confirmation image to be obtained, after the first target set is centered. Observers will
require confirmation images of all target sets, and so such a capability would be used
frequently. The work-around is to schedule separate target sets as individual visits,
but that entails unnecessary guide star acquisitions and MSA reconfigurations that
can be avoided.
• Some science observations need to ensure that there are no interruptions in the data
taking for a single target set. A “NON-INT” special requirement may be needed.
• The onboard scripts now assume that exactly the same sequence of actions occur at
every dither location in a visit. At each dither location, the scripts can loop through a
list of gratings each with their own exposure time. However, the scripts (currently) do
not have a mechanism for specifying different MSA configurations or different
exposure times at different dither locations.
• The assumed mode of operation for NIRSpec is an observation planned well in
advance and with a preliminary NIRCam image so that precise source positions can
be measured as part of preparing for the target acquisition. Targets of opportunity
(TOOs) cannot be observed that way (with possible rare exceptions) and so an
alternative acquisition scheme is needed. In general, it should be possible to acquire
reliably TOOs if they are observed in the 1.6 arcsec square fixed slit and if there is a
peak-up algorithm available to tune the pointing.
• Planet transit data-taking is likely to place unusual demands on scheduling the
observatory. It may be necessary, for example, to tolerate high overhead times so that
a visit can execute reliably at the correct time and so that an instrument is in a
predictable and stable configuration in order to obtain very high signal-to-noise data.
These overheads need to be accounted for, at least statistically, in evaluating
proposals.
• Exoplanet observations are likely to need sub-arrays because of the source brightness.
• For exoplanet studies it is necessary to obtain a very large number of short
integrations to both reach the needed S/N and to cover the time period in question.
Such a situation may require splitting the observation into several exposures to work
around observatory constraints, but those separate exposures must be timed to ensure
continuous data during the eclipse.
• To achieve very high S/N it may be advantageous to suppress re-acquisition of guide
stars.
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4.2
Documentation and user information (including APT):
• Users will need better information than is now available for the overheads associated
with changing guide stars, or with dithers to cross the detector gap.
• A program that both dithers (as virtually all will) and uses multiple gratings can be
carried out in several ways, and it is not clear which method is most efficient or most
conservative of resources. Some simulations may be beneficial and better
information is needed.
• Some types of observations involve high dynamic range scenes in which faint objects
are near to bright ones. This may cause problems if bright objects fall on closed
shutters of low contrast, and there is a need for such shutters, when known, be
identified. In addition, in such scenes it may be necessary to take detector persistence
into account in constructing the flow of observations.
• Despite the many shutters available in the MSAs, the potential for overlapping
spectra, the distribution of target centers relative to shutter centers, and the limited
dynamic range of a single exposure mean that multiple target sets are needed for even
a modest list of objects. Some guidance to users in this area would be helpful.
• The currently available information on detector readout (NRSRAPID versus NRS) is
not adequate to make an informed judgment. A quantitative calculator may be
needed.
• The software that helps to optimize centering of targets may benefit from a user being
able to note whether individual targets are point sources or not because extended
objects generally do not need precise centering in a shutter to get good data.
• Measuring the positions of faint objects on a preliminary NIRCam image that are near
bright objects is compromised by the saturation present.
• Exoplanet hosts are likely to be very bright and so it may be difficult to measure their
positions relative to reference stars well on a preliminary NIRCam image. The
shortest possible full-frame NIRCam image will saturate. The peak-up procedure
used for centering a TOO would also work well for bright targets.
• It is unclear to a user how much overhead time is associated with changing guide
stars, as when a large-scale dither is used to cross the detector gap.
• The interplay of dithering and changing gratings can cause confusion. Is there a
preferred hierarchy or order? Do we wish to prefer one method over another to
reduce mechanism usage?
• In some cases observers may wish to obtain preliminary NIRCam images at more
than one wavelength so that backgrounds can be evaluated. Information on this will
be needed.
• Very crowded fields are problematic. Some simulations of such observations may be
helpful in guiding users.
• Observations of very bright sources may be challenging for reliably rejecting cosmic
rays because of the smaller number of groups obtained.
• It is not clear if APT can handle the construction of IFU mosaics. The relation of
such mosaics to guide stars is also unclear, and that then means overhead times are
undefined.
• Observers will need guidance in comparing different modes for obtaining spectra
over large areas with either the MSA or IFU.
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5.0 Conclusions
We anticipate further development of these scenarios as useful “test particles” for various
aspects of STScI systems. The above lessons learned will affect decisions on how
commanding for NIRSpec is written, for example. These scenarios can be expanded and
refined to provide input to a test “Call for Proposals” as a test of those systems, too.
Additional scenarios are being written for additional science cases, and, particularly, to
include moving targets and how those will be acquired and observed. Some aspects of
observation planning mentioned above – such as NIRCam preliminary images – will
require detailed examination so that we understand such things as the lead time required
and necessary precisions. These scenarios also give us a view to the kinds and methods
of information needed by users in preparing proposals and programs, another effort that
will be undertaken in the near future.
6.0 References
Garcia Lopez, R., Nisini, B., Eislöffel, J., Giannini, T., Bacciotti, F., & Podio, L.
2010, A&A, 511, 5.
Lu, J. R., Ghez, A. M., Hornstein, S. D., Morris, M. R., Becklin, E. E., and Matthews, K.
2009, ApJ, 690, 1463
Simon, T., & Dahm, S. E. 2005, ApJ, 618, 795
Stiavelli, M. 2009, “Observational Cosmology with the ELT and JWST,”
in Science with the VLT in the ELT Era, Ap. & Sp. Sci. Proc.
Tanvir, N. R. et al. 2009, Nature, 461, 1254.
Wang, H., Yang, J. Wang, M. & Yan, J. 2002, A&A, 389, 1015
Ward-Thompson, D., Zylka, R., Mezger, P.G., & Sievers, A. W. 2000, A&A, 355, 1122.
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Appendix: Details of the programs
7.0 Program 200: Kinematics of stars in the Galactic center
Project title:
Kinematics at the Galactic Center
Author:
J. Valenti
Purpose:
A test case of observing many point sources in a crowded region to obtain
radial velocities of giant stars in the Galactic Center to measure the mass
of the central black hole and to separate populations kinematically.
Finding valid background shutters may be difficult. Multiple pointings are
needed to cover a sufficiently wide field to achieve the science goals.
SI usage keywords:
Crowded field; multiple pointings
Instrument modes:
NIRSpec
MSASPEC
G235M+F170LP
NRSRAPID
7.1
Scientific background:
Lu et al. (2009) have recently published an image of the Galactic Center obtained with
Laser Guide Star Adaptive Optics (LGS AO). Their Figure 5 is reproduced below. The
two panels show the same field at different scales, with Sgr A* located at the black cross;
this is the presumed location of the Galactic Center. The arrows show the direction and
magnitude of measured proper motions, with red arrows denoting objects that Lu et al.
believe to be members of a disk, and blue arrows are for non-members. The objects
range from 9 to 15 in K magnitude, and coordinates are good to 2 mas precision. The
green slitlets are to scale but are otherwise schematic to illustrate the difficulty of locating
unoccupied background in this crowded region.
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This second image has additional slitlets added to represent a second configuration and
pointing so as to observe additional stars in the same field.
Slitlets shown in green constitute one target set and slitlets shown in purple constitute a
second target set. The two target sets are observed sequentially, not simultaneously. In
general, a given field of view may have multiple target sets. By default, the MSA
planning tool will select slitlets in a single target set such that spectra formed by the
slitlets will not overlap. This scientific constraint means that a horizontal line anywhere
in the figure intersects at most one green slitlet and at most one purple slitlet.
Note that the targets in the green slitlets are all in the top shutter of each slitlet. This is
one of the two dither locations required for the green target set. The second dither
location is offset by 0.45 arcsec along the vertical axis of the figure, such that the targets
move to the same relative position in the bottom shutter of each slitlet. The MSA
configuration does not need to be changed between these two dither locations.
The visit break down table below shows that each target set will also have a third and
fourth dither location (not shown in the figures), offset by one shutter along the
horizontal axis. These are dithers in the sense that the targets are identical, but the MSA
configuration for dither positions 1 and 2 is different from the MSA configuration for
dither positions 3 and 4.
When switching from the green target set to the purple target set, an MSA
reconfiguration is required and a small target offset is required to position the new set of
targets in the top shutter of the purple slitlets. Between the first and second exposure of
the purple targets, a dither of 0.45 arcsec along the vertical axis is required. Two more
dither positions (not shown in the figure) are required, as with the green target set.
7.2
Required MSASPEC template parameters:
o Spacecraft pointing and orientation:
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o Five sets of [MSA RA, MSA Dec, MSA Orient] values; one per FOV.
o Target acquisition:
o Filter, MSA configuration, and readout pattern (Ngroup = 3, NINT = 1, subarray
= FULL).
o Filter = F110W, readout = NRSRAPID.
o Acquisition reference stars list (RA and Dec): 5 sets.
o Ten MSA configurations for acquisitions; two per target set.
o Dither pattern: x-offset, y-offset, MSA configuration.
o 40 sets of [x-offset, y-offset] and 20 MSA configuration values.
o Grating+filter list, for each dither position:
o G235H+F170LP.
o Confirmation image for each grating+filter combination:
§ Readout pattern = [NRS, NRSRAPID], NGROUP = [0,3] (NINT = 1,
subarray = FULL).
Science
spectrum parameters for each grating+filter.
o
§ Readout = [NRS, NRSRAPID], NGROUP = [3, 6, 8, 10, 12, 13, 15],
NINT = [1, 3].
7.3
Observation notes:
o Observations:
o Visits are very short, so group all visits into a single observation
o Target sets (distinct sets of targets):
o Some target sets are too bright for a confirmation image
o Initial pointings for each target set are offset by a small amount
§ Fraction of a shutter to align target set in shutters
§ Integer offset to minimize the impact of failed shutters
o Four dither positions: 2 x 2 block of adjacent shutters
§ Measuring velocities, so no need to fill wavelength gap
DMS
will process each target set as a separate association
o
o Target acquisitions:
o Pointings for all target sets and all dithers in a visit are within 5”
§ Only one target acquisition is needed for a visit
o Configure the MSA during target acquisition to block bright stars
Exposures:
o
o One grating for all exposures
o NRSRAPID is used when NRS would yield fewer than 6 groups
o Targets in set 3 are so bright that only 3 groups are possible
§ Set NINT=3 to allow cosmic ray rejection
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7.4
Visit breakdown:
7.5
Observing concerns identified:
o Rules need to be established to allow the insertion of multiple target acquisitions
within a single visit. There are several scenarios that require that capability:
o There is a need for a re-acquisition to be made after a significant time has
elapsed (nominally 10,000 sec).
o A re-acquisition is also needed after a dither slew of more than ~5 arcsec.
o Different science programs may have different requirements on pointing
tolerance.
There
is
a need to obtain a confirmation image for each target set within a single visit,
o
not just the first. This is a capability that would be used very frequently, in fact as a
default. A potential work-around exists if separate visits are specified, but at a
significant cost in additional time for guide star acquisitions that are not really
necessary.
o The Data Management System (DMS) requires observers to specify details of data
reduction in their Phase II programs. However, as far as the observatory it self is
concerned, these are only comments.
o It is not clear if a single set of acquisition parameters is sufficient for all the target
sets in a visit.
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8.0 Program 201: Evolution of ices in star-forming environments
Project title:
Evolution of ices in star-forming environments
Author:
T. Beck
Purpose:
Instrument modes:
8.1
NIRSpec
MSASPEC
G235H+F170LP; G395H+F290LP
NRSRAPID
Scientific background:
This program would observe NGC 2024, an embedded star-forming region (SFR) in
Orion. NGC 2024 has about half a dozen B-type stars in the central region and the
extinction seen has a large gradient, from AV ~ 5 at the periphery to AV ~ 30 at the center.
The B stars in the center are optically invisible because they are obscured by a tongue of
dense cloud material.
The goal is to get accurate continuum measurements of point sources to measure ices in
the cloud material.
8.2
Observation notes:
o The observations use G235H+F170LP and G395H+F290LP with NRSRAPID
readout.
o A confirmation image is required to verify target centering.
o There are four target sets that require 4 pointings and 4 guide stars.
o There are 4 exposures and MSA configuration per target set:
o Two spatially distinct configurations for dithering, with possible
reconfiguration of the MSA.
o The wavelength gap needs to be filled with an additional dither of about 20
arcsec.
o Observations within a target set should not be interrupted.
o The acquisition needs to take account of very bright sources in the field, both as a
protection risk and because those objects will saturate in the preliminary NIRCam
image.
Guide star
1
1
1
2
2
3
Target set
Exp. (grating 1)
1
2
3
4
5
6
30
60
200
30
100
30
Exp. (grating 2)
30
60
240
40
160
40
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Guide star
3
4
4
4
Target set
Exp. (grating 1)
7
8
9
10
100
20
50
200
Exp. (grating 2)
160
30
60
240
8.3
Observing concerns identified:
o Like other scenarios, this program requires a confirmation image to be obtained after
each MSA reconfiguration, not just the first one that follows the target acquisition.
o It is not clear to a user how much overhead time is associated with changing guide
stars, as when a large dither is made to cross the detector gap.
o It is not clear how to make the program most efficient given that multiple dithers are
needed for two different gratings. Is it best to dither first and then change gratings?
o It is strongly desired to not be interrupted during the observations for a given target
set. Should there be a Special Requirement?
o The brightest targets in the central cluster have K = 5 and will saturate in a NIRCam
image. This may cause problems if such bright objects fall on low-contrast shutters.
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9.0 Program 202: Jets from young stars
This example of an observing scenario by D. Karakla helps to illustrate the process and
results:
Project title:
YSO jets near IRS1 in NGC 2264
Author:
D. Karakla
Purpose:
Test case of observing highly extended objects. These are also emissionline objects that cover a broad range of flux levels.
9.1
SI usage keywords:
Extended objects; bright objects; point sources
Instrument modes:
NIRSpec
MSA
G140M+G235M (extended sources)
G140M+G235M+G395M (point sources)
NRS+NRSRAPID
Description:
YSO (Young Stellar Object) jets are emission-line sources found in star-forming regions.
Many are also designated as Herbig-Haro (HH) objects. One well-studied star-forming
region is the ~5-Myr cluster NGC 2264, which is about 760 pc distant. HH objects are
jets emanating from very young stars and as such are extended primarily in one direction
along an axis. Molecular hydrogen (H2) emission (1.121 micron and more) is often seen
in jets with lower- to intermediate energy levels, excited by shocks in outflows, along
with [Fe II] lines (1.257 microns and more), although not necessarily in exactly the same
spatial locations. The knots in these emitting regions have velocities of 100–200 km s–1.
The illustration below is from Simon and Dahm (2005) and it shows the region of interest
and several features. This image was made from J-, H-, and K-band images using
QUIRC on the 2.2-m telescope on Mauna Kea. EXS-1 is an x-ray flare star, and the
contours shown are from an observation made with XMM-Newton. IRS-1 is a very
strong infrared source discovered with the IRAS satellite. Note the faint knots.
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The next illustration (below) shows infrared data for jets in NGC 2264 from Wang et al.
(2002). These were obtained at the Okayama Observatory in J, H, K’, and H2 (2.121
microns). The five sources marked with a cross are likely to be high-mass proto-stars
(Ward-Thompson et al. 2000).
The goals of this program are to study the emission knots of YSO jets as well as the
potential YSO sources in this region. The knots are emission line sources, and the
brightest knots appear as red objects in 2MASS K-band images and in Spitzer IRAC 3.6micron images. Spectra are to be obtained of not only the jets but also the YSOs
presumed to drive the jets. It may also be possible to find the infrared counterparts to the
strong millimeter sources in this area.
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9.2
Exposure time estimation:
No fluxes or spectra were found in the literature for these sources. However, the
brightest knots are seen in 2MASS K-band images and the limiting K magnitude in those
is ~16.5. Similar sources were found and scaled to the distance of NGC 2264.
Likely exposure times were estimated as follows:
• For the knots in the jets, the emission-line spectrum shown in Garcia-Lopez et al.
(2010) was used, taken to extend over 10 MSA shutters, with 2 additional shutters
used to measure background.
• The Exposure Time Calculator (ETC) of J. Valenti was used with these flux values to
estimate an exposure time of 212 sec for an integrated (total line flux) S/N = 4 in one
of the weaker [Fe II] lines at 1.6 μm in Band 1. About 170 sec is needed to achieve
S/N = 4 in a weak H2 line at 2.248 μm in Band 2. Final exposure times were adjusted
upward.
• The jets may arise from some fainter YSOs in the field which are also to be observed,
as are near-infrared counterparts to 5 known millimeter-wave (MM) sources (WardThompson et al. 2000).
9.3
MSA planning:
• It is assumed that a preliminary image with NIRCam will be obtained and from that
precise source positions will be measured.
• The HH objects (jets) are expected to be extended primarily in one direction (along
their axis). However, these jets also extend in the cross direction as well, and that
may require additional MSA configurations to provide full spatial coverage.
• A test case of MSA usage was based on existing astrometry. It was found that several
separate target sets were needed to achieve the science goals and to achieve optimal
centering of sources in the shutters; also they were organized hierarchically by
brightness so that any one target set can be observed well with a single exposure time.
Version 17.0.3 of APT includes a prototype MSA planning tool.
The figure below shows this field in Aladin, as taken at 3.6 microns with IRAC on
Spitzer. It would be necessary to close shutters during the target acquisition in order to
avoid saturation by the brightest objects. The NIRCam image would probably best be
done in the narrow-band H2 filter in order to minimize saturation and to locate the knots.
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9.3.1 Observing summary:
• A narrow-band NIRCam preliminary image is needed to obtain precise astrometry of
sources. Additional NIRCam images may be needed to estimate background fluxes.
• NIRSpec confirmation images are required.
• The operating mode is MSASPEC:
o G140M+F100LP (Band 1) to observe [Fe II] lines at 1.257 and 1.644 μm at R
= 1000.
o G235M+F170LP (Band 2) to observe H2 at 2.121 μm at R = 1000.
o Both use NRS readout mode, except use of NRSRAPID when the number of
NRS groups falls below 6 (for better cosmic ray rejection).
• Three target sets are needed to cover all extended sources in this scenario:
o One guide star may suffice for all 3 target sets, but an additional target
acquisition is needed to dither objects across the detector gap.
o At the same time, the durations of exposures prevent placing all exposures
into a single visit.
o Each target set needs 12 exposures plus 2-4 MSA configurations to achieve
the needed spatial coverage of knots:
§ There are 3 spatial offsets of the MSA pattern, using slitlets that are 16
shutters high on the larger knots (10 source shutters and 2 for
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•
9.4
background, with 2 shutters closed between the end of source and the
background at top and bottom).
§ A large dither is done to fill the wavelength gap.
§ This is all done for both gratings.
For the point sources, a 3-high by 1-wide slitlet is adequate for each. A specific setup was not tested, but the total number of point sources is small and it is assumed that
they can be accommodated in one or possibly two target sets.
o The K-band flux of ISR1 (Simon & Dahm 2005), another YSO showing a
bipolar outflow, was scaled to K-band fluxes of sources as measured by WardThompson et al. For the three bands:
§ 201 sec to get S/N > 10 in Band 1.
§ 53 sec to get S/N > 10 in Band 2.
§ 53 sec to get S/N > 10 in Band 3.
§ NRSRAPID is used because the sources are bright.
§ Gratings G140M, G235M, and G395M are used at R = 1000.
§ Complete wavelength coverage is desired, and so dithering is used to
cover the wavelength gap between the detectors.
§ There are 36 exposures per target set.
§ Confirmation images are required.
Summary for extended sources:
Exposures per dither point for extended objects, using a 16-shutter slitlet. Pattern uses 3
spatial and 2 spectrum dither points for 3 target sets.
Guide star
Target set
1
1
1
1
1
1
1
1
2
2
3
3
Grating+filter
G140M+F100LP
G235M+G170LP
G140M+F100LP
G235M+G170LP
G140M+F100LP
G235M+G170LP
Exposure (sec)
296 (NRS, 7)
296 (NRS, 7)
222 (RAPID, 21)
222 (RAPID, 21)
466 (NRS, 11)
466 (NRS, 11)
Note that for target sets 2 and 3 an attempt was made to vary the exposure times up and
down to allow for sources both 50% brighter and fainter.
9.5
Summary for point sources:
Exposures per dither point for point sources, using a 3-shutter slitlet. Pattern uses 3
spatial and 2 spectrum dither points for 1 to 2 target sets.
Guide star
Target set
1
1
1
1
Grating+filter
G140M+F100LP
G235M+G170LP
Exposure (sec)
212 (RAPID, 20)
212 (RAPID, 20)
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Guide star
Target set
1
1
1
1
1
2
2
2
Grating+filter
Exposure (sec)
G395M+F290LP
G140M+F100LP
G235M+G170LP
G395M+F290LP
63
212
212
63
(RAPID,
(RAPID,
(RAPID,
(RAPID,
9.6
Observation concerns identified:
• The existing NIRSpec templates are minimally sufficient to support this kind of
observation, but significant efficiency improvements could be made.
• A significant and unanticipated number of separate target sets were found to be
necessary for a given pointing, even with only a few dozen total objects. This was
needed to cover the full area, to be able to deal with a broad range of source
brightnesses, and to optimize source centering in shutters and also to avoid
overlapping spectra.
• It is not always easy to judge when to trade off use of NRS versus NRSRAPID.
NRSRAPID is preferred when there is a risk of losing too much data due to cosmic
rays (below about 250 sec exposure time), but the default use of NRSRAPID raises
data volume concerns.
• It may be helpful to allow the optimization code to take account of the type of object,
based on the NIRCam pre-image. Point sources must be centered in the sweet spot of
a shutter, while extended sources do not, but the extended sources must still avoid bad
shutters.
• Overlapping spectra can be a problem in even a moderately-crowded field.
• The MSA Planning Tool will need to provide a capability that avoids placing bright
object “spoilers” in low-contrast shutters.
• The optimization of the MSA configuration will depend critically on the quality of
the NIRCam pre-image.
• Measuring the positions of sources near bright objects will be affected by saturation.
• It is not now possible to obtain more than a single confirmation image in a visit.
Multiple target sets can be observed, but observers will need the means to obtain a
confirmation image for each.
• Currently, the onboard scripts assume that exactly the same sequence of actions occur
at every dither location in a visit. At each dither location, the scripts can loop through
a list of gratings each with their own exposure time. However, the scripts (currently)
do not have a mechanism for specifying different MSA configurations or different
exposure times at different dither locations.
10.0 Program 203: Formation of massive stars
Project title:
Massive star-forming regions in the Milky Way
Author:
J. Muzerolle
Purpose:
Very crowded regions with a wide brightness range.
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Instrument modes:
10.1
NIRSpec
MSASPEC
G140M, G235M, G395M
NRSRAPID
Scientific background:
The figure below shows a NICMOS image (F110W+F160W+F222M) of the region W3.
Enlargements of an area at two longer wavelengths are also shown.
The goal of this program would be to determine star formation rates and the initial mass
function (IMF) in a cluster containing massive stars, where there is an environment of
extreme radiation and very high densities. The evolution of circumstellar disks would
also be studied.
The information to be obtained would include spectral types, measures of spectrum
veiling or continuum excess, and signatures of accretion and/or outflows, such as atomic
and molecular emission features.
The stars to be observed number in the hundreds per cluster, and range in K magnitude
from about 11 to 18 at a nominal distance of 2 kpc.
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10.2 Operations notes:
0.0 The figure below shows examples of MSA fields for the bright (left) and medium
(right) targets. The stars in red actually get observed, and blue objects would be
saturated.
o A confirmation image is required.
o There are 4 exposures per target set so as to obtain spatial dithers:
o One dither is done to cover the wavelength gap, and that requires two MSA
configurations per target set.
o There would be two open shutters per target in each configuration, with a
dither performed between the two shutters.
o There would be multiple target sets to deal with the range in brightnesses,
from bright (11<K<14) to medium (14<K<16) to faint (16<K<18), with
exposure times of ~30, 100, and 1000 sec for S/N > 50 in G140M and
G235M, and S/N = 30 in G395M.
o To estimate exposure times a catalog of known members of the Orion Nebula Cluster
was used, scaled to a distance of 2 kpc. This is a typical distance for the nearby
massive star-forming regions, and the resulting field of view is well matched to the
MSA. There would be ~1500 potential targets in the desired brightness range.
o An optimization code was used to determine MSA configurations. This included the
dither to cover the wavelength gap. Three configurations were used for the three
ranges of brightness noted above to provide spectra of 136/319 bright stars, 215/674
,medium, and 157/477 faint stars, which means about 30-40% of potential targets are
observed.
o Note that if no gap dither were used then only one configuration per target set would
be needed and it would be possible to observe 40 to 50% of the potential targets.
o The total time needed for 9 target sets in one cluster is about 9 hours.
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10.3
Exposure breakdown:
Guide star
Target set
targets/cand G140M
idates
(sec)
G235M
(sec)
G395M
(sec)
1
1
59/319
30
30
30
1
2
41
30
30
30
1
3
36
30
30
30
1
4
79/674
100
100
100
1
5
72
100
100
100
1
6
64
100
100
100
1
7
56/477
1000
1000
1000
1
8
54
1000
1000
1000
1
9
47
1000
1000
1000
10.4 Observing concerns identified:
o The organization and scheduling of target sets:
o Faint stars may need different acquisition break points.
o The observations may need to be chained in such a way that bright sets are
done first since they can accommodate the initial slew “tax” and still stay
within the maximum visit size of 10,000 sec.
o There are concerns over background subtraction in this crowded region, particularly
at longer wavelengths (above 3 microns). Pre-imaging at multiple wavelengths will
be needed.
o Dealing with source crowding in order to get reliable background subtraction, as well
as guide star and reference star selection.
o There are ~20 sources in the field that will saturate. In some cases it may be possible
to detect them through closed shutters, particularly for the longer exposures to study
the faint stars. A mechanism for tracking such objects may be needed.
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11.0 Program 204: First-light galaxies in the Hubble Ultra-Deep Field
Project title:
First-light galaxies in the Hubble Ultra-Deep Field
Author:
D. Soderblom
Purpose:
A few extremely faint galaxies have been found in the Hubble Ultra-Deep
Field with large photometric redshifts. Spectroscopic redshifts of these
objects are needed to confirm their distances. These objects may be the
faintest observed by NIRSpec and so test the ability to schedule repeated
visits with the same MSA configuration so as to build up the needed signalto-noise.
Instrument modes:
11.1
NIRSpec
MSASPEC
G235M+F170LP; G140M+F100LP
NRSRAPID
Description:
This scenario is based on SO-DRM program 402 as written by M. Stiavelli in January,
2006. The goal is to obtain medium-resolution spectra (R = 1000) to confirm the nature
of first-light sources in the distant universe. For a brief discussion of the science, see
Stiavelli (2009).
The objects to be observed are extremely faint galaxies in the UDF. These objects are
marginally resolved, with sizes of 0.2 arcsec or so, requiring careful placement in the
NIRSpec MSA shutters for good throughput. The z850 magnitudes of the objects of
interest (i-band drop-outs) range from about 27 to 29.5, with most fainter than 28 (see
Fig. 1 in the above ref.). The J110 magnitudes are about 27 to 29.
Using the conversion provided by Jakobsen (“Calculating the nominal sensitivity of
NIRSpec”), which is ABmag = 31.43 – 2.5 log(F(nJy)), then J110 = 27 corresponds to a
flux of about 60 nJy, and 29 to about 10 nJy.
The exposure time calculator (ETC) rendered in IDL by Tumlinson predicts that one
needs 6.4 Msec for the 10 nJy sources at 1.2 microns, or nearly 1800 hours. The brighter
(60 nJy) sources would need about 300 hours.
By comparison, the SO-DRM allowed for two sets of exposures:
300 hours at R = 1000 and the medium wavelength setting (G235M + F170LP).
100 hours at R = 1000 and the short wavelength setting (G140M + F100LP).
For this exposure time, the scenario in the SO-DRM only reaches the brighter objects
shown in Stiavelli’s paper. Whether this is a problem or not (in reaching a JWST
primary science objective) requires judgment by experts in the study of first-light
galaxies.
Note that these galaxies are presumed to be largely continuum sources and not
necessarily strong sources of Lyman-alpha line emission. The goal in obtaining spectra is
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to detect the Lyman break and so confirm photometric redshifts so that the galaxies can
be more precisely set into the cosmic time-scale.
The anticipated density of the primary targets is about one per square arcmin, or ~10 over
the NIRSpec FOV. These objects are extremely faint, and In addition, other relatively
faint objects will be observed, presumed to be galaxies at z > 6. The density of these
objects is taken to be several per square arcmin and they are assumed to be brighter,
needing 10 hours integration each.
Because of the very long integration times, this program is broken into 10-hour visits.
Each visit will always observe the same 10 first-light objects, plus an additional set of
other objects that changes for each visit. In addition, this program may execute at
varying roll angles, requiring additional MSA configurations. A total of 40 visits is
needed.
11.2
Visits and sub-visits
Each 10-hour visit is 36 ksec long and so is broken into four 9 ksec units. Each visit uses
a different MSA config.
Each 9 ksec unit (sub-visit) starts with a target acquisition and consists of 9 x 1 ksec
exposures. This then allows for nine dither positions. Some dithers are sub-aperture and
use the same MSA config, others go to different apertures.
11.2.1 Middle-wavelength sub-visit
Parameter Value Opsmode
Optical elements
MSASPEC
G235M + F170LP
Readout
Conf. image?
NRS
Yes
MSA configs
3
Exposure
1,000 sec per dither
Dither
3x3
11.2.2 Short-wavelength sub-visit
Parameter Opsmode
Optical elements
Value MSASPEC
G140M + F100LP
Readout
Conf. image?
NRS
Yes
MSA configs
3
Exposure
Dither
1,000 sec per dither
3x3
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11.3
Scheduling of observations
The proposed observations of the HUDF pose a distinct challenge that few other
programs will match: their extended duration. The total time needed is 400 hours, or 40
visits of 10 hours each, which means a total of roughly 20 days of on-target time.
Is it possible to observe the HUDF at a single roll angle for 20 days? A single roll would
be preferred because then one could be certain of getting the same faint, high-redshift
galaxies. A change in roll angle is likely to mean some object would no longer fall
within the sweet spot of a shutter.
The plot below, prepared by W. Kinzel, shows the maximum permitted time at a given
angle in ecliptic coordinates. For the HUDF it turns out that the maximum at one
orientation is 13 days.
However, one can achieve the same roll six months later, and so a total of about 26 days
should be available at a single roll angle for the HUDF pointing. If such a strategy is
used, differential velocity aberration may lead to positional shifts within the MSA fields
that cause objects to fall outside their sweet spots. In other words, it may not be good
enough to execute two separate pointings six months apart. Another possibility is to
execute this program over two years.
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12.0 Program 205: Carbon abundances in Omega Centauri
Project title:
Carbon abundances in Omega Centauri
Author:
J. Tumlinson
Purpose:
Test case of obtaining observations of an extremely densely populated
field.
SI usage keywords:
MSA
Instrument modes:
NIRSpec
MSASPEC
NRSRAPID
12.1
Description:
This scenario has been constructed to test the feasibility of NIRSpec MSA observations
in an extremely crowded field, in this case near the center of the Galactic globular cluster
Omega Centauri. The full description of this scenario is appearing in a separate report
and so will only be briefly summarized here.
This field offers more than 24,000 stars brighter than R = 20, a brightness at which the
high-resolution grating in NIRSpec yields signal-to-noise = 20 in 400 sec. About half of
those stars randomly fall within the acceptable sweet spot of MSA shutters.
The observing strategy used in this study is to keep the telescope fixed at a single position
and to then repeatedly reconfigure the MSAs to move across the field to useful targets.
This strategy assumes no spectrum overlap and no need for background measurement at
these bright levels.
12.2 Operations notes:
o A total of 40 separate MSA configurations and exposures are needed, for a total of
16,000 sec or 4.4 hours, before overheads are included.
o Some shutters with good targets must be avoided because they end up with more than
one good target.
12.3 Observing concerns identified:
o Low-contrast shutters can potentially spoil observations for other objects in the same
row, but this appears to not be a significant problem in this study.
o NIRSpec can successfully observe this challenging field.
13.0 Program 207: MSA spectroscopy of a very extended object: NGC 6302
Project title:
MSA spectroscopy of a very extended object: NGC 6302
Author:
T. Keyes
Purpose:
Test case of obtaining NIRSpec observations of a very large object (3 x 3
arcmin) without information on background or target acquisition. The
efficiencies of alternate scenarios are compared.
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SI usage keywords:
Extended object; mosaics
Instrument modes:
NIRSpec
MSASPEC
G140H+F100LP
NRSRAPID
13.1
Description:
NGC 6302 is a Galactic planetary nebula with a large physical extent on the sky (>~3 x 3
arc min). This program is intended to study nebular abundances and the thermal and
ionization structure at high spatial scale using diagnostic emission lines in the NIRSpecaccessible IR. The scenario considered uses NGC 6302 as a proxy for observations of
any object whose physical extent is comparable in size to the MSA field, and for which
no contemporaneous background and target acquisition observation is possible.
The observing strategies considered in this study compare two alternatives:
• Keep the telescope fixed at a single position and repeatedly reconfigure the MSAs to
move across the field.
• Use a single column of shutters and move the telescope in a sequence of slews to
move the observation sampling across the field.
These strategies assume no spectrum overlap will occur and that there is no need for
background measurement.
The image on the left below is an HST ERO image issued recently. Narrow-band WFC3
images are available using filters at H-alpha, [N II], [O II], and [O III]. These reveal
numerous shocks, filaments, and outflows extending nearly a parsec from the central star.
Near-infrared emission lines arising from a broad variety of species are also present. The
right-hand image shows NGC 6302 overlaid with the outline of the NIRSpec 3 x 3
arcmin field of view, with the yellow line representing a full-length MSA column of
shutters.
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13.2 Observing notes:
o An initial goal is to obtain spectra that can reveal a wide range of ionization states and
species, particularly high-ionization lines, as well as [Fe II] and H2 features.
o No target acquisition is required (and none may be possible), but there is a need to be
able to identify positions accurately after the fact. Hence a confirmation image with
the MSA open would be obtained and is probably preferable to a NIRCam image.
o The grid of points on the nebula would be obtained by using an entire vertical column
of open shutters, stepped 480 points at 0.25 arcsec each. Thus the entire field would
be about 180 x 120 arcsec. Once the 4-point dithers are counted, a total of 1,920
exposures is obtained in the spatially-scanned direction.
o The exposure taken at each step is short, roughly 3 min, with dithers included.
However, the effects of saturation for this bright source have not been considered and
some special exposures may be needed in some cases.
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o The total time needed is estimated at ~24 hours, but that is before guide star
acquisition times are included. The number of guide stars needed to cover this region
is not known.
13.3
Use of the MSA versus the IFU:
For this large object the intent is to obtain a full set of spectra parallel to the disk axis.
This could be done using either the MSA or the IFU, with these considerations (see also
program 261):
• The MSA provides poorer spatial sampling in the dispersion direction: about 0.2
arecsec compared to 0.1 arcsec for the IFU.
• At the same time, the IFU would need about 6 times the number of pointings.
• MSA failed-open shutters can cause significant problems for this or similar programs,
effectively making an entire row useless.
• Use of the MSA means the need for fewer guide stars compared to the IFU.
• Both modes require dithers to treat detector effects, but failed-open shutters may
require additional dithers to work around them.
13.4 Observing concerns identified:
• Information on the need for preliminary and/or confirmation images is insufficient at
this point to judge their need. For this program, it would be ideal to have both before
and after images, even during the raster sequence.
• It is not clear how mosaic tiles will be registered for later combining. Will FGS
information be adequate for this purpose? Will it is necessary to obtain a NIRCam
image?
• This object and ones like it have inherently have a very large range of brightnesses.
The problem of detector persistence may require division of the observations into
stages defined by dynamic range. This may exacerbate registration concerns, and is it
possible to keep such a sequence of operations within a single visit?
• As noted with program 261, it is not yet clear how mosaics and the need for guide
stars interact. Will this be automated?
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14.0 Program 230: NIRSpec follow-up of Gamma-ray burst afterglows
Project title:
NIRSpec follow-up of Gamma-ray burst afterglows
Author:
J. Tumlinson
Purpose:
Test case of obtaining NIRSpec observations on a quick-turnaround basis.
Tests ability of the system to handle TOOs.
SI usage keywords:
Quick-turnaround TOO; faint sources; fixed slit
Instrument modes:
NIRSpec
FIXEDSLIT
G235M+F170LP
NRS
14.1
Description:
JWST is required to be able to respond to targets of opportunity (TOOs):
• Regular TOOs do not interrupt the current Observation Plan and need only be
executed within two weeks.
• Rapid turn-around TOOs require much faster response, with this requirement:
o “The S&OC is required to be able to update the executing Observation
Plan to incorporate a Rapid Response TOO within 24 hours after
submission of the updated TOO Observation (MR-293); this depends on
the availability of a DSN contact to intercept the executing Observation
Plan.” (JWST-STScI-000648)
o The 24-hour requirement breaks down into 22 hours for the PPS to process
the new Phase II submitted by the PI and to perform the necessary safety
and feasibility checks, plus 2 hours for the new Observation PLan to be
staged by the FOS for upload when the next ground contact occurs.
o This implies the following time budget for response to a Rapid Response
event:
§ User completion of new Phase II program using APT: 2–4 hours.
§ Processing by PPS: <22 hours.
§ Staging and upload: < 2 hours.
§ Wait for next ground contact at two per day: <12 but typically ~6.
o This makes the likely shortest time to be 32 hours (2 + 22 + 2 + 6) and the
maximum about 40 hours (4 + 22 + 2 + 12).
These net turn-around times (32 to 40 hours) allow for an adequate response to a gammaray burst event. For example, GRB 090423 still had a flux of ~5 mJy that late after
outburst. However, events at higher redshift will presumably be fainter, and it would
make success much more likely if the 22-hour processing time – the single biggest item
in the time budget – could be reduced. In addition, the above calculations do not really
take account of the limited on-shift hours for JWST operations. An event triggered on,
say, Friday evening could be delayed much more.
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14.2
Scientific background:
Gamma-ray burst (GRB) afterglows can be used as distant light sources to reveal
absorption from gas in the host galaxy or in the intervening inter-galactic medium (IGM).
The host galaxy gas may contain tracers of molecular gas or diagnostics of ionization,
temperature, and metallicity. The spectrum from a GRB is flat and that of a synchrotron
source resulting from a reverse shock in the ejected material.
GRBs are now being detected and announced rapidly and automatically by the Swift
satellite. This rapid-turnaround capability has proved to be so important that it is likely
that successors to Swift will be operating in the JWST era.
The illustration below shows the evolution of the spectral energy distribution (SED) of
GRB 090423, taken from Tanvir et al. (2009). The estimated redshift is 8.2. The three
bands distributed in frequency on the left of the figure correspond to K, H, and J in going
from left to right. The observations are in time sequence from top to bottom. Note that
Tanvir et al. obtained S/N ~ 3 over 400 Å bins in 2.5 hours using VLT with SINFONI.
The NIRSpec ETC indicates that NIRSpec can obtain S/N = 15 per resel at R = 1000 in
~10 min.
14.3
Operations notes:
In most respects a GRB afterglow is an ordinary single-object, fixed-slit observation.
The goal is to obtain a reduced one-dimensional spectrum over 1 to 5 microns at good
signal-to-noise. However, GRBs offer a significant challenge because of their transitory
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nature. In particular, it is not feasible to obtain a preliminary image with NIRCam, and
so the target acquisition must be done with less-precise coordinates. Several questions
arise:
• Can a satisfactory acquisition be achieved in, say, the largest fixed aperture? Would
that require additional software similar to the ACQ/PEAKUP operation performed
with COS on HST?
• How are TOOs handled by the ground system and is that system adequate to meet
science needs?
• Will there need to be observing “taxes” levied on observers seeking rapid-turnaround
observations such as the 15-orbit penalty assessed on HST observers of quickturnaround TOOs.
Regarding acquisitions, the default target acquisition strategy for NIRSpec has been
developed with the MSA in mind, given that it is anticipated that MSA observations will
dominate NIRSpec usage. That TA strategy places targets within an acceptance zone of
an MSA shutter with an accuracy of 20 mas, given coordinates of sufficient precision.
This process requires the use of 8 to 10 reference stars with relative astrometry good to
~5 mas, based on NIRCam or other equivalent imagery. In comparison, a GRB’s
position can be good to ~100 mas, based on immediate ultraviolet or optical follow-up,
but such positions do not come with any reference stars.
An alternative, as noted, is to use the 1.6 arcsec square fixed aperture in NIRSpec,
together with flight software that can scan a somewhat larger area of the sky than the
aperture itself. Another possibility is to use reference stars but to allow coarser
requirements on their relative astrometry, perhaps using information from 2MASS,
VISTA, or SASIR.
14.4 Observing concerns identified:
• The assumed mode of operation for NIRSpec is an observation planned well in
advance and with a preliminary NIRCam image so that precise source positions can
be measured as part f preparing for the target acquisition. TOOs cannot be observed
that way (with possible rare exceptions) and so an alternative acquisition scheme is
needed. In general, it should be possible to acquire reliably TOOs if they are
observed in the 1.6 arcsec square fixed slit and if there is a peak-up algorithm
available to tune the pointing.
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15.0 Program 231: Exoplanet atmospheres
Project title:
Exoplanet atmospheres
Authors:
J. Valenti, D. Long
Purpose:
Test the ability of the system to handle observations that must be executed
at a specific time in order to capture an event.
SI usage keywords:
High signal-to-noise; critical timing
Instrument modes:
NIRSpec
FIXEDSLIT
NRSRAPID
15.1
Scientific background:
Exoplanet observations have yielded some of the top science results from Hubble and
Spitzer, and the same is likely to be true of JWST. JWST will observe exoplanets
transiting in front of the host star and half an orbit later being eclipsed by the host star.
For astrophysical context, the table below gives median and extreme characteristics for
66 known transiting planets. The number of planets known to transit stars brighter than
K=12 will increase by a factor of a few between now (mid-2010) and the launch of
JWST.
Property
Minimum
Median
Maximum
Ks magnitude (2MASS)
5.5
9.85
14.7
Orbital period (days)
0.8
3.2
111
Transit duration (hours)
0.8
3.1
12
Transit depth (%)
0.035
1.1
2.8
Eclipse contrast ratio at 3.9
microns
1E–6
8E–4
2E–2
Observers will use two complementary observing strategies to characterize exoplanet
atmospheres. The eclipse spectroscopy strategy subtracts spectra obtained during eclipse
from spectra obtained immediately before and after eclipse. The subtraction removes
direct emission from the star, leaving only starlight reflected by the planetary atmosphere
and thermal emission from the planet. The transit spectroscopy strategy divides spectra
obtained during transit by spectra obtained immediately before and after transit. The
division yields the fraction of starlight not blocked by the planet, which is a function of
wavelength because opacity in the planetary atmosphere is a strong function of
wavelength. Eclipse and transit spectra will yield planetary radii, composition (e.g.,
water, methane, carbon monoxide, carbon dioxide), vertical temperature structure
(including stratospheric temperature inversions), and horizontal heat distribution.
Planet atmospheres have spectral features that are typically 0.01% (transit) to 0.1%
(eclipse) of the stellar signal, so observations must achieve a S/N ratio of 10,000 per hour
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and per wavelength bin. JWST has more than enough sensitivity to achieve this S/N ratio
for dozens of transiting exoplanets. The challenge will be to minimize systematic errors
due to mechanical, thermal, and electronic transients on timescales of minutes to hours.
15.1.1 Observing description:
Transiting planets have well-defined ephemerides. Most known exoplanets have orbital
periods of a few days, implying tens of scheduling windows per year. In general, visits
will not have an orientation constraint. Given the flexibility of event-driven observations,
the observatory may be ready to execute an exoplanet visit before the nominal start time.
The observer should be charged (at least statistically) for the time spent by the
observatory waiting for the exoplanet transit or eclipse to occur.
As soon as the preceding visit completes, the observatory should slew from the old
attitude to the exoplanet-observing attitude. [N.B. A dummy visit could perhaps be used
to force an early slew, if the standard visit execution logic is not sufficient.] Slewing
immediately allows slew-related mechanical and thermal transients to decay as much as
possible before the high-precision exoplanet observation begins. If it is not too difficult
to implement, extra exposures of the target should be obtained after target acquisition and
before the nominal visit start time. Starting exposures immediately allows the detectorrelated electrical and thermal transients to decay as much as possible before the highprecision exoplanet observation begins.
Observations should use the large (1.6 x 1.6 arcsec) fixed slit. For a well-centered star,
this large aperture is relatively insensitive to target drift (due to ISIM thermal changes
and/or rotation about the FGS guide star) because the wings of the PSF have dropped
roughly symmetrically to relatively low levels at the edge of the aperture. [N.B. What
level of displacement is acceptable without affecting photometric precision?]
The baseline NIRSpec target acquisition procedure requires precise coordinates for the
science target, relative to a set of reference stars distributed across the NIRSpec field of
view. NIRCam pre-imaging is used to obtain precise relative coordinates. However,
exoplanet hosts are bright enough to saturate NIRCam, even in the shortest possible fullframe exposure. In JWST-STScI-1751, Beck describes an alternate strategy for acquiring
bright objects (particularly exoplanet hosts) in the large aperture. This strategy should be
implemented and used for exoplanet observations.
Exoplanet observations with any of the gratings will use the (yet to be defined) S1600A1
subarray, which spans 32 rows and 2048 columns per detector (0.66 seconds per frame).
Observations of the brightest exoplanet hosts may require fewer columns (less
wavelength coverage) or fewer rows (less precise photometry) to avoid saturation.
Exoplanet observations with the PRISM and F070LP filter will use a subarray that is 32
rows by 512 columns (0.17 seconds per frame) because the prism spectrum does not fill
the detector. [N.B. Smaller subarrays may also be needed for the other fixed slits.]
Because exoplanet hosts are bright, integrations will saturate after only a few frames.
The NRSRAPID detector pattern (one frame per group, i.e. NFRAMES=1) will be used
to record multiple groups before saturation. Because the data rate for a subarray is 25%
the data rate for full-frame, NRSRAPID can be used indefinitely without exceeding the
NIRSpec data volume allocation.
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Whenever possible, integrations will have at least 4 up-the-ramp measurements per
integration (NGROUPS), so that the pipeline can use the standard algorithm to detect
cosmic rays and fit ramps. For the brightest sources, NGROUPS will need to be less than
4 to avoid saturation. Cosmic rays can still be detected by comparing count rates in
successive integrations. Even NGROUPS=1 may yield valuable data for bright sources,
despite a lack of information about the bias level after each pixel reset.
Exoplanet observations will typically be twice as long as the transit duration plus extra
time for slew, setup (e.g., guide star acquisition, NIRSpec target acquisition), and
detector stabilization. An exoplanet with a 3 hour transit would be observed with the
following sequence:
45 min
• Slew:
30 min
• Set-up
Stabilization:
30 min
•
90 min
• Pre-eclipse:
180 min
• Eclipse:
90 min.
• Post-eclipse:
From the perspective of photometric stability, the data gathered during stabilization
through post-eclipse should be a single 390 minute exposure with 390*60/0.66 = 35454
integrations. This may exceed the maximum length for a single exposure and/or the
maximum number of integrations for an exposure. This means that the observation may
need to be split into multiple exposures. To minimize the effect on the photometry of
exposure boundaries, exposures will preferentially be started well out of eclipse or at the
midpoint of the eclipse. In the example above, a plausible set of four exposures would
have durations of 60, 150, 150, and 30 minutes with 5455, 13636, 13636, and 2727
integrations, respectively.
Nominally, a new target acquisition is required after 10,000 seconds to compensate for
target drift. This implies a target acquisition before each of the four exposures described
above. However, for very precise photometry of exoplanets, the mechanical, thermal,
and electronic disturbances associated with a target acquisition may create larger errors
than the drift between target acquisitions. Depending on actual observatory performance,
exoplanet observers may need to suppress target acquisitions. Note that splitting out of
eclipse observations into a pre-eclipse and a post-eclipse segment corrects to first order
the effects of target drift.
15.2 Observing concerns identified:
• Because of the need for exact timing of the observations, planet transit data-taking
is likely to place unusual demands on scheduling the observatory. It may be
necessary, for example, to tolerate overhead times so that a visit can execute
reliably at the correct time and so that an instrument is in a reliably stable
configuration in order to obtain very high signal-to-noise data. These overheads
need to be accounted for, at least statistically, in evaluating proposals.
• Exoplanet hosts are likely to be very bright and so it may be difficult to measure
their positions relative to reference stars well on a preliminary NIRCam image. The
shortest possible full-frame NIRCam image will saturate. The peak-up procedure
used for centering a TOO would also work well for bright targets.
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• Exoplanet observations are likely to need sub-arrays because of the source
brightness.
• Observations of very bright sources may be challenging for reliably rejecting
cosmic rays because of the smaller number of groups obtained.
• For exoplanet studies it is necessary to obtain a very large number of short
integrations to both reach the needed S/N and to cover the time period in question.
Such a situation may require splitting the observation into several exposures to
work around observatory constraints, but those separate exposures must be timed to
ensure continuous data during the eclipse.
• To achieve the needed S/N it may be advantageous to suppress re-acquisition of
guide stars.
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16.0 Program 261: Atomic hydrogen filaments in Perseus A (NGC 1275)
Project title:
Atomic hydrogen filaments in Perseus A (NGC 1275)
Author:
T. Beck
Purpose:
Test case of obtaining observations of a large object at high spatial
resolution, requiring a large number of mosaicked exposures.
SI usage keywords:
IFU; mosaic pattern
Instrument modes:
NIRSpec
IFUSPEC
G140H
NRSRAPID
16.1
Description:
Perseus A is a cD galaxy at the heart of the Perseus cluster of galaxies. This program is
intended to study the kinematics and energetics of the neutral hydrogen filaments. The
figure below shows an HST/ACS image of Perseus A, together with an indication of the
3-arcsec square NIRSpec IFU field of view.
16.2
•
•
Operations notes:
This program will use the IFU on the (assumed) need for 0.1 arcsec spatial
resolution, and also for better rejection of unwanted light from other parts of the
object.
The goal is to observe both Paschen-alpha and Paschen-beta in a single setting,
with additional features from [Fe II] and H2 appearing.
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•
•
•
The overall mosaic is 484 p[ointings in a 22 x 22 pattern on 2.7 arcsec centers in
order to get 10% overlap between pointings.
Each IFU position in the mosaic will be done with a 4-point small pattern. At 30
sec each, each position should need about 3 min, including overheads.
The total integration time is about 23 hours, and that does not include the time
needed for guide star and target acquisitions. The pattern is shown below.
• It is estimated that 5 guide stars will be needed for 5 sets of targets.
16.2.1 The IFU compared to the MSA:
This program’s goal is to obtain full 3D spectra of the inner 60 x 60 arcsec region. The
MSA could be used instead of the IFU, but with these consequences:
• Poorer spatial sampling in the dispersion direction because of the 0.2 arsec-wide
shutters.
• Dithering in the cross-dispersion direction would be needed to work around the
MSA bars.
• A full 60 arcsec of slit is needed with no failed shutters.
• About 300 pointings would be needed in the dispersion direction to sample the full
60 arcsec.
At the same time, use of the MSA would likely mean that fewer guide stars would be
needed because stepping would be done in only one direction, not two.
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16.2.2 Observing concerns identified:
• Can APT handle IFU mosaics like this and how will they be defined?
• How does a mosaic deconstruct into guide star needs? Will that be automated?
How does that translate into visits?
• If an observer cannot tell how a large IFU mosaic breaks down with regard to guide
stars then they cannot calculate the overheads. There may be a need to use average
values to estimate the observing time request.
• It is not clear how to balance different modes for obtaining spectra over large areas,
using either the MSA or IFU. Some guidance for observers will be needed.
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17.0 Program 502: MIRI/NIRSpec IFU observations of extragalactic H II regions
Project title:
MIRI/NIRSpec IFU observations of extragalactic H II regions
Author:
K. Gordon
Purpose:
Study the properties of the spectroscopic features of poly-cyclic aromatic
hydrocarbons (PAHs) in extragalactic star-forming regions and their
dependence on the ionization and radiations fields as well as the shape of
the extinction curve in the ultraviolet.
SI usage keywords:
IFU; mosaic pattern; multi-instrument
Instrument modes:
NIRSpec
IFUSPEC
G395H+F290LP, G140H+F100LP
NRSRAPID
17.1
Description:
This program would use both NIRSpec and MIRI to obtain complete spectrum coverage
from 1.7 to 28.2 microns, and would use the IFU modes in both instruments stepped in
mosaics across the target to provide full spatial coverage as well. The putative target is
the nearby galaxy M 101.
The spectra would be used to study features of PAHs in star-forming regions and how
those features depend on local metallicity and on the local radiation fields. Also, the
shape of the dependence of interstellar extinction on wavelength in the ultraviolet is
different in different places, and these observations would be used to potentially connect
those extinction curve changes to PAHs. The PAHs are tracers of the physics of the dust.
In additional, atomic and molecular spectrum features would be used as indicators of star
formation, abundances, and warm H2.
What is known now is that the strengths of PAHs correlate better with the hardness of the
radiation field than with metallicity. This implies that the carriers of PAHs are being
modified or destroyed by the radiation rather than being formed from, say, enhanced
abundances. At the same time, the strengths of PAH features are constant up to a
threshold hardness and then decrease in strength as the radiation hardens.
The targets for this program would be about a dozen well-studied regions in M 101 that
have measured metallicities.
17.2
Observing notes:
This program presents several challenges:
• Many pointings are needed to cover regions that are near each other. These
pointings should be done with the same overall observing set-up so that the spectra
obtained can be compared directly.
• Targets should be clustered for efficient scheduling.
• Scheduling efficiency is also boosted by obtaining both NIRSpec and MIRI data at
each IFU position before moving the telescope. This minimizes overhead times.
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The overall observing strategy would run as follows:
• Map a 10 x 10 arcsec region at each pointing.
§ This ensures that a sufficient region is mapped to mitigate sensitivity
to small-scale variations within a star-forming region.
§ This also enables the study of such small-scale variations.
§ The JWST PSF at 25.5 microns will have a FWHM of about 0.9
arcsec.
• NIRSpec and MIRI IFU “mimi-maps” would be obtained.
• These require dithers and a mini-map in order to get the same spatial coverage
at all wavelengths.
• NIRSpec requires two grating settings, with a third as well if the 1.0–1.7
micron band is desired.
• MIRI will require three grating settings simultaneously through the four IFUs.
• Use of the MIRI Imager in a short exposure with F560W will allow for
precise registration of the dithers.
• The MIRI IFU fields change in size, growing with increasing wavelength because
of the larger PSF. The smallest IFU field is 3.7 arcsec square in Channel 1. A 3 x 3
mini-map with this IFU will cover 10 arcsec square with 10% overlap.
• At each tile position, a 4-point dither pattern would be carried out to get good
spatial sampling and some redundancy. Only a 2-point dither is needed to get
adequate spatial sampling for all the IFUs because of the design of the image
slicers. The data obtained will be somewhat undersampled spectrally.
• With NIRSpec, a 10 x 10 arcsec square region would also be mapped. The
NIRSpec IFU covers 3 x 3 arcsec at each pointing, and so a 4 x 4 mimi-map is
needed to cover 10 arcsec square with 10% overlap.
• The NIRSpec grating settings would include:
• G395H+F290LP to cover 2.9–5.0 microns and to obtain 3.3 microns.
• G235H+F170LP to cover 1.7–3.0 microns and to obtain Paschen-alpha at
1.87 microns.
• G140H+F100LP (possibly) to cover 1.0–1.i8 microns.
Each
position would have four dither points to cover the spectrum gap and to sample
•
spatially.
The exposure times needed are short. Target fluxes range from 0.12–15 mJy (3.6
microns); 0.11–25 mJy (8 microns); and 2–923 mJy (24 microns). With MIRI, most
targets need only 4 x 12 sec. In FASTMODE this translates to 4 groups. With NIRSpec,
4 x 30 sec expoures should suffice, or 3 groups in NRSRAPID mode.
17.3
Observing sequence:
Based on the above, the following sequence of observations would occur:
• First, it is not clear if a target acquisition is needed, given pointing accuracy and the
regions covered.
• With the MIRI IFU:
• Guide star acquisition.
• 3 x 3 mimi-map, with 4 dithers at each position.
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Three grating settings, with a full mini-map executed at each grating position.
108 total exposures with both MIRI IFU detectors (short- and long
wavelength).
• A possible additional 108 MIRI IMager F560W pre-images to enable precise
registration.
With the NIRSpec IFU:
• Guide star acquisition.
• 4 x 4 mini-map, with 4 dithers at each position.
• Two grating settings, with a full mimi-map executed at each grating position.
• 128 total exposures with both NIRSpec detectors.
• There is no anticipated need for NIRCam images to register the NIRSpec
observations.
•
•
•
17.4 Observing concerns identified:
• Use of cluster targets in templates would boost observing efficiency significantly.
Observing 10 targets without the use of cluster targets would use about 5 hours, but
with cluster targets that are near each other that drops to about 1 hour.
• A single visit should contain multiple dithers and mini-maps. For instance, a 10 x
10 arcsec map should not require additional guide stars.
• The APT mosaic tool would be very helpful for constructing the mini-maps.
• This program may not need a formal target acquisition.
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