TECHNICAL
REPORT
Title: Further Definition of the NIRSpec
Science Templates
Authors: D. Karakla, T.
Beck, W. Blair, C. Keyes, J.
Muzerolle, K. Pontoppidan,
D. Soderblom, J. Tumlinson,
and J. Valenti
Phone: 410
338-4947
Doc #: JWST-STScI-002288, SM-12
Date:
03 November 2010
Rev:
Release Date: 18 March 2011
1.0 Abstract
This technical report describes the APT templates for the NIRSpec science instrument
modes. An earlier document by Fullerton (2008) first described the JWST templates.
Since then, a new Bright Object (BO) template has been added to enable the high priority
science of planetary transits. We describe our concept of how NIRSpec observers will be
able to operate the instrument and use tools to assist them in choosing appropriate
parameter values. The templates will surely evolve as we gain a better understanding of
the instrument from ground testing, and as our concept of how to operate the instrument
and plan observations evolves.
For example, NIRSpec observations will generally require a TA. However, we will not
include a description of alternate TA strategies (other than a tweak to the Standard TA
using the MSA), since the operational details are being discussed now. In the future, the
FS and IFU templates will allow alternate TA methods for certain science needs, and the
BO template will use a particularly designed (non-standard) TA method by default.
Clearly, the PPS templates will also have to capture pertinent information from the
observer related to the specific observations to not only carry out the observations, but
also instruct the DMS on the appropriate reduction of acquired data. Many details of the
mechanisms for doing so are undefined at this time, but work is ongoing in the instrument
team to flesh out the details and derive requirements.
Throughout this report, we highlight (in blue) new capabilities needed in APT. In all
template tables presented here, parameter default values are underlined. For reference,
we have included an Appendix of common acronyms used throughout the paper.
2.0 Introduction
NIRSpec observations will be specified using an observation form in APT. The
observation form will include an instrument template specific to each instrument mode. A
NIRSpec “Observation” will likely use a single instrument mode: the IFU, MSA, or FS,
(TBC), and will consist of one or more visits at the same orientation. Each visit will use
one guide star and must include only one guide star (GS) acquisition, but may need to
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have more than one target acquisition. Visits may be separated by a minor slew, but large
pointing changes (of a size TBD) within an Observation, are not permitted.
Many NIRSpec observations will consist of several closely spaced target sets in a field of
interest. It is possible they may share the same GS and may even be grouped into a single
visit if exposure durations are short enough. Several of the NIRSpec examples outlined in
a report on Observing Scenarios by Soderblom (2010) use this approach, which is
described in relation to Multi-Object Spectrograph (MOS) observations in Section 3.6.
For a more complete description of NIRSpec observing terms (e.g. field and target set)
and their definitions, see Chapter 7 of the latest version of the NIRSpec Operations
Concept Document (OCD) (Böker and Valenti 2010).
Phase I and II templates for each mode are indistinguishable at this time. This will likely
change at least for NIRSpec MOS observations. Observation forms must allow those
programs with sufficient orientation and timing flexibility to fully define their
observations early so that these programs can be inserted into the Long Range Plan (LRP)
to be able to estimate the overheads for observers (and the TAC). However, more
complex programs – those with visit links or MSA observations awaiting NIRCam preimages - will need to use a two-phase approach.
Due to the geometry of the sunshield and the orbit of the telescope, there's a limited
window at which a particular aperture position angle (PA) can be observed for any target.
The size of the window varies depending on the ecliptic latitude. The PPS will need to
know what appropriate orientation ranges are possible for each observation.
After the TAC decides on approved programs and an initial LRP is generated, the PIs will
receive a finite range of orientation angles (thus, a corresponding range of dates) for their
observations. PIs will then be able to better define their observations (e.g. specify exact
MSA configurations). However, some NIRSpec observations (like many IFU or FS
observations) may be feasible without requiring an exact orientation. The PPS should be
able to handle planning these less-restricted observations. For example, it may be
possible to successfully design a program of IFU, or even MSA, observations without
knowledge of the precise orientation if the target is symmetric. More generally though,
NIRSpec observers will benefit from the observing paradigm discussed in Section 3.6,
which addresses MOS observation strategies. These include most MOS, and many IFU
observations.
3.0 NIRSpec Templates Definition
An observation form will be used to design a NIRSpec observation in APT. An
Observation may contain one or more visits. A visit is limited to using a single instrument
mode, and instrument templates define the instrument parameters needed for that mode.
Users will not be asked to design visits per se, but the visit structure, including the
insertion of GS and target acquisitions, will be determined by APT after users specify
their observations and will be displayed to users upon request. A template will be used to
specify observations in a single field (which may include multiple targets and dithers)
containing one or more visits, but users will have additional tools in APT to quickly
duplicate observations (or their components) and make small changes.
There are three observing modes with NIRSpec: Fixed Slit (FS) spectroscopy, Integral
Field Unit (IFU) spectroscopy, and Multi-Object spectroscopy (MOS) with the microshutter array (MSA). Entrance apertures for all modes are located in the plane of the
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MSA. All three modes use the same filter wheel and grating wheel assemblies and
operate over the spectral range from 0.7 to 5.0 µm. Parameters for each mode differ
enough that we have defined separate templates for each. An additional special purpose
template will be described; A large square aperture (1.ʺ″6 x 1.ʺ″6) has been approved and
adopted by the NIRSpec science team for the high profile extra-solar planet transit
science case. Bright object observations are high priority, but are fundamentally different
from observations of faint objects. A separate template will simplify the planning of
bright object observations. The NIRSpec science instrument templates are:
§ MSA template
§ FS template
§ BO template
§ IFU template
The Target and TA specification forms are discussed separately in Sections 3.1 and 3.2.
The recommended TA strategy will depend on the science goals and instrument
configurations. The default strategy, designed to give the highest precision, is described
in the NIRSpec OCD (Böker and Valenti 2010) and involves imaging 8 to 20 reference
stars with the MSA and measuring their centroids within 32-pixel square detector subarrays. Alternate TA strategies are planned for some FS and IFU observations.
Parameters for those TA strategies will be discussed in a future version of this document.
As a prerequisite for most MSA observations, users will additionally complete a NIRCam
template as part of their program. NIRCam pre-images and NIRSpec science
observations will probably be linked Observations of the same program. The NIRCam
templates are described in a similar report (Sahu 2010a). The MSA observations will
likely occur two to six months after the NIRCam pre-images are obtained; Scheduling is
governed by source visibility limitations due to orientation restrictions and the need to
provide observers enough time to design MSA configurations using accurate source
positions derived from the pre-images.
APT will need to assist users in defining large or complex NIRSpec observations, and
manage constraints common to all JWST observations (Many of the detailed
requirements of these remain to be defined.):
• Help the user design the most efficient layout of visits to minimize GS acquisitions.
• Ensure guide star availability for each visit.
• Limit total exposure durations to some TBD value. (A hard upper limit of 10 ksec is
•
•
•
•
imposed by the pointing stability of the high gain antenna.)
Ensure compliance with other visit duration limits (TBD). The observatory will need
to adjust its momentum about once a day in a momentum management maneuver, so
the maximum visit duration limit is 24 hours. Science observations of faint targets
could extend beyond this limit as planned, so the PPS will need to break long data
requests into smaller visits.
Insert TAs where needed when a visit must be split. (This is TBD.)
Estimate instrument, acquisition, and observation overheads for the observer.
Assist users by providing tools to quickly and easily repeat a sequence of dithers N
times (where N is large) for all NIRSpec instrument modes. This is a common
observational strategy for IR observations.
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The shutters of the MSA each subtend an area of just 200 x 450 mas on the sky. An MSA
Tool has been created for the precise specification of open and closed shutters to assist
observers in designing MSA observations. The tool, described in Section 3.7, will help
optimize the pointing of the MSA, and select TA reference stars for each pointing.
3.1 Target Specification Form
In this report, “Target” applies to a set of RA and Dec coordinates that define a given
pointing. In the case of FS and IFU observations, the science target is also the Target for
pointing the telescope. Table 1 lists the parameters to specify a Target. For MOS
observations, the “Target” is a reference point centered between the four MSA quads.
This point is neither within the science aperture nor imaged onto the detector. The Target
is not an MSA science target. Hence, “Target Flux” is irrelevant for MOS observations.
Instead, a “saturation flux” specified in the MSA science template will be used to derive
appropriate MSA exposure times which will help prevent saturating sources of interest.
For the IFU and FS modes, though, Target Flux as specified in the Target Template is
used to determine exposure times, readout cadence, and other observational parameters.
Except for some very special science cases, TA reference stars will be required. Each TA
reference star’s position and flux will need to be specified using the TA Specification
Form discussed in the next section. (TA reference star fluxes will be used to derive an
appropriate TA exposure time, but is not used to refine pointing during the TA itself).
3.1.1 Moving Targets
Observers planning to observe individual moving targets will probably plan to use the
1.″6 square aperture or the IFU. The assumption is that MSA spectroscopy will not be
done on moving targets. Certainly, TA would be challenging, and MSA observations of
multiple moving targets would place large demands on the planning system.
Table 1 The Target Specification
Input
Details
Identification
Target Number
Target Name
Target Class
Alternate Names
Default
…
…
Fixed, Generic, Moving,
Offset
…
Target Type (astrophysical)
…
...
Target Description
Position
…
RA (ICRS) (J2000)
…
Dec (ICRS) (J2000)
Proper Motion
Target Flux
1
1
0.ʺ″1
0.ʺ″1
0.ʺ″0 /year
0.ʺ″0 /year
…
…
RA Uncertainty
Dec Uncertainty
RA PM
Dec PM
Epoch
Waveband (J, K, or M) or
wavelength
Remarks
Programmatic reference
From a preferred catalog
Menu
[Optional]
Multiple entries permitted
Menu
Menu: depends on target
type
Target or Pointing
position (MSA center)
Target or Pointing
position (MSA center)
[default for ICRS]
[default for ICRS]
[units are TBD]
[units are TBD]
Required for IFU and
Fixed Slit Observations
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Input
Details
Default
Remarks
only.
Flux (magnitude) or Surface
Brightness (mag/as2) or other
(consistent with ETC options)
…
Depending on target type
…
Flux Units
Flux Uncertainty
…
…
Other Fluxes [multiple entries]
Optional
1
Information
…
Radial Velocity
0.ʺ″0
…
0
Parallax
Comments
Ephemeris
2
Info
SemiMajor Axis (AU)
Eccentricity
Inclination (degrees)
Longitude of Ascending Node
(degrees)
ArgPerihelion (degrees)
Mean Anomaly (degrees)
Equinox
Epoch
1
µJy (or convert from other
units]
More information on
spectral energy
distribution
In km/s, or as a redshift
Text block
0
0
0
0
0
B1950, J2000
dd-MMM-yyy:HH:mm:ss
or
YYYY.DDD:HH:mm:ss
Menu
This field should not appear in an MSA Observation Template.
2
Ephemeris info fields should be editable in moving target programs only, consistent with HST
methodology.
3.2 Target Acquisition Specification Form
NIRSpec spectroscopy requires a TA to accurately place the target into a slitlet or
aperture. Depending on the science, varying levels of accuracy may be needed. The
standard method uses the MSA and a selection of reference stars. The NIRSpec TA is
required to have the same orientation as the science data.
3.2.1 Standard TA Using the MSA
The standard TA is an on-board procedure that involves taking acquisition images with
the MSA. These images are taken with three reads in the readout pattern derived based on
reference star brightness. Three reads are needed to remove cosmic rays in an on-board
procedure. The extracted TA reference stars are centroided and the measured coordinates
are transformed to sky coordinates and compared with positions measured on the preimage. A calculated corrective slew is then commanded to the telescope. The process is
repeated until the residual rms error is <20 mas. For all NIRSpec modes, this is the
default TA strategy, and all templates will offer it.
The standard TA method must be used for all MSA observations. It can also be used with
other NIRSpec instrument modes if the user requests the IR pre-image to use this method,
or if the positioning accuracy given by this method is needed (e.g. FS and IFU
observations in crowded fields). Parameters required for this TA are listed in Table 2.
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Only two filters are offered for TA; the broader filter is for fainter reference stars. The
selection of TA reference stars using the MSA Tool is described in Section 3.7. Since
centroids are measured on-board, it is necessary for the planning software to ensure that
the reference stars will not saturate the detector in the first three reads. The NIRSpec
OCD gives limits of AB=20.6 (or 19.6?) for the F140X filter, and AB=18.3 for F110W.
Table 2 The TA Specification Form (Standard Method)
Field
Value(s)
Description
Preliminary
Direct Image
MSA TA
Configuration
File
Filename
NIRCam, ACS, or WFC3 pre-image. Required for the standard TA method.
Filename
[Optional] The file that describes the status of each shutter when the MSA is
in Protected mode. In Open MSA mode, no file should be specified. The
presence of a filename implies MSA Mode = Protected.
TA Filter
F140X,
F110W
F140X = Extra-Broadband B filter for TA at 1.4 µm
F110W = Broadband A filter for TA at 1.1 µm
TA Readout
Pattern
NRSRAPID,
NRS,
NRSSLOW
Filename or
(RA, Dec,
Flux in µJy)
for each star
entered into a
GUI
Required to determine the integration time of the TA image. The default
readout pattern will be derived from reference star brightnesses. (Rules to be
provided by the WIT team.) The TA image will always consist of 3 reads.
[Optional] A file listing the reference stars to be used for TA. There should be
a minimum of 8, and a maximum of 20 reference stars. For each star, the RA,
Dec, and flux need to be specified. Typically, stars will be selected using the
MSA Tool. Optionally, a user may enter values into a GUI form, but these
must be checked with the MSA Tool once a final MSA position has been
determined.
Reference Stars
To protect the detectors from bright stars in the field, an optional MSA TA
Configuration File describing the open/closed status of MSA shutters must be specified.
The MSA Tool (Section 3.7) will be used to design this file.
Other TA strategies may be used if pre-imaging is not possible, or not warranted by the
level of accuracy required, or if the default strategy is operationally infeasible. These
other strategies are discussed in detail in Beck (2009). When implemented, these methods
will be made available with the FS or IFU science template. The standard TA described
above will be the default.
Nominally 10,000 seconds after the initial TA, a new TA will be needed to update the
pointing. The rules for inserting additional target acquisitions will be complicated and
may depend on science goals, especially when dithering within a single visit, e.g. to clear
accumulated offset errors associated with the positional uncertainty in dithering. The
proposal system (with guidance from the observer, TBD) should determine where in the
observing sequence to insert additional target acquisitions if a visit extends beyond the
allotted time interval.
A TA Reference Image is automatically taken at the end of the TA process to verify the
positions of the reference stars. It is a direct image (Grating MIRROR in place), taken
with the same settings used for the TA: the TA filter, MSA configuration, exposure time,
and readout pattern.
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3.2.2
Alternate TA Strategies for FS (Including the 1."6 Square Aperture) and
IFU Observations
Alternate TA strategies have been defined for situations where we know the standard TA
will fail. Some of them do not involve the MSA. These first need to be vetted by
commanding before addressing how the templates will be affected.
However, some observations with the 1.″6 square aperture FS and the IFU will be
possible with less accurate centering than provided by the standard TA method discussed
above. (A notable exception is planet transit observations which will require ~20 mas
positional accuracy for stable photometry.) If the required centering accuracy can be
relaxed enough, pre-imaging may not be needed. In this case, a confirmation image
(Section 3.6.1) is recommended after a coarse TA. Here we discuss the Coarse Accuracy
TA method proposed by Beck (2009), which is a slight modification to the standard TA.
- Coarse Accuracy TA for IFU observations, and 1."6 square aperture observations
of faint targets (including faint Moving Targets). A modified standard approach
may be used with reference stars in the MSA, but with catalog coordinates measured
from the SDSS (r < 20), 2MASS (J < 16), or other catalogs, provided the astrometric
accuracy of coordinates is 80 mas or better. Observers must specify at least three to
five reference stars in this case. No pre-imaging is required. Target brightness in the
1."6 square aperture is limited to targets that will not saturate the detector during
full-frame readout in imaging mode (limits are TBD).
The parameters needed for specifying a Coarse Accuracy TA are the same as those for
Standard TA shown in Table 2, with these exceptions:
1) A minimum of three to 5 TA reference stars is needed for coarse TA, rather
than 8. As for the standard TA, reference stars will typically be chosen using
the MSA Tool while simultaneously ensuring that the Target is centered
within the science aperture (either the IFU or 1."6 square aperture).
2) The catalog used for coarse TA should be recorded for reference in the
proposal. If catalogs other than the SDSS or 2MASS are specified, the
observer should be warned that the accuracy required for TA is at least 80 mas
or better (Refer to Beck 2009, for more information.)
3.3 Fixed Slit Template
There are five fixed slits for NIRSpec. Three are narrow, long slits (~0."2 x 3."5); one is a
little wider (~0."4x 4."0); and the last is the 1."6 x 1."6 large square aperture, which will
be used for the high-profile extra-solar transiting planet observations. This 1."6 square
aperture will also be used for observations of objects of various brightness that don’t have
strict limitations on target centering, or perhaps poorly known coordinates. Note the large
square aperture was added late in 2008, and so was not considered in the preliminary
report on PPS templates by Fullerton et al. (2008). The Fixed Slits are located in the
plane of the MSA. Fixed slit spectroscopy with NIRSpec will, by default, use subarrays.
These were extracted from the NIRSpec OCD (Böker and Valenti 2010), and included in
Table 3 below for convenience.
Table 3 NIRSpec Sub-Arrays for FS Spectroscopy
Default
Subarray Name
Associated Slit Name/Description
Starting
Row†
Height in
Rows†
Width in
Columns†
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ALLSLITS
S200A1
S200A2
S200B1
S400A1
S1600A1
General use subarray that spans the
entire spatial area of all fixed slits
(Rows 890 - 1145)
A_200_1 Medium slit lower edge of
fixed slit area (0."2 x 3."5)
A_200_2 Medium slit nearest IFU
(0."2 x 3."5)
B_200 Medium slit upper edge of fixed
slit area (0."2 x 3."5)
A_400 The wide slit (0."4x4."0)
A 1."6 x 1."6 square aperture.
890
256
2048
911
64
2048
951
64
2048
1061
64
2048
993
966
(TBR)
64
2048
64
2048
† Values are TBR.
If the target is bright enough to saturate at any observed wavelength in the grating used,
then the user should specify this in the template and provide a peak target brightness flux
to avoid saturation (namely, a “Saturation Flux”). Depending on the target spectrum,
saturation could occur at different flux levels in different gratings, so the user can choose
either to provide the peak flux at the brightest part of the spectrum, or elect to use the
ETC results for the source or those of another source. A splinter group is studying how
best to interface the ETC to APT to assist the observer in determining parameters that
will be used to derive exposure times. Assuming that peak brightness will be returned
from the ETC results, it will be converted by APT into a Saturation Flux, and inserted
into the corresponding field in the templates. This flux will be used to compute the
number of integrations needed to attain a desired exposure time without saturation
(Karakla et al. 2009).
For very bright sources, additional sub-arrays proposed by Tumlinson (2009a, Table 1)
have been accepted for high-priority NIRSpec planetary transit science. These will be
made available only with the bright object template and are discussed in relation to that
template in Section 3-5.
Parameters required for the specification of FS observations are shown in Table 4. For
each fixed slit, the user will select from TA strategies appropriate to that slit, and the
default detector sub-array will be configured from the slit. If selected, ALLSLITS will
override the default. Several targets may be observed in the same visit with the same slit.
For each target, the user may choose several grating and filter pairs. The exposure time is
derived from the requested exposure time and selected readout pattern. If the saturation
flux of the target in a grating/filter pair is defined, the derived exposure time will depend
on the computed number of integrations as well.
Table 4 Inputs to the NIRSpec Fixed-Slit Spectroscopy Template
Input
Value
Details
Remarks
Specify an entrance aperture.
(See Table 3)
Choose from menu.
A. Visit Level Parameters
Fixed Slit
menu
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Input
Value
Sub-Array
Default, or
ALLSLITS
Standard,
TA Method
Coarse
Accuracy
Details
Remarks
Default configured from the
chosen slit. ALLSLITS is
optional, but is required when
dithering across the gap.
Both choices need TA
reference stars to be defined.
Coarse accuracy should be
selectable only when using the
Sub-arrays for FS are defined in
Table 3. The same sub-array
applies to all exposures in a
Visit.
TA strategy to be used
depending on accuracy needed
for science.
1."6 square aperture.
B. TA Specification
See Table 2
C. Target Specification (Allow multiple Targets per FS selection)
Target Name/ID
Specify a target
Choose from menu
D. Instrument Configuration (Allow multiple Dispersers per Target)
Dispersion Elements
menu
Specify grating and filter
combination
Choose from menu
E. Exposure Specification
Saturation Possible?
Yes/No
Peak flux below which target
should not saturate. Will help
determine the number of groups
in a integration (Ngrp) and the
integration time (Tint). ††
Together with saturation flux,
this will help determine the
number of integrations in an
exposure (Nint), and a final
Exptime.
Saturation Flux
Requested Exptime
Readout Pattern†
NRSRAPID,
NRS
Ngrp†
Nint†
Can be a Radio Button
1, or a
positive
integer
Max Ngrp=1000 (NRSRAPID)
and 250 (NRS).
= 1 for faint targets when
saturation is not an issue.
Derived. Number of integrations
in an Exposure.
See Table 5
Yes/No
Exposure time in seconds (per
dither step)
A Default will be derived. If
default is NRSRAPID, user
should be allowed to select
NRS.
Derived. Number of groups in a
integration.
(Value will determine Tgrp.
Rules for minimum Ngrp are
TBD).
Exptime†
Dither Pattern
Options
Special
Requirements
Offered only if user indicated
saturation is possible. Enter
value at wavelength of
observation, or choose to use
ETC.
Specify via generic form
Derived exposure time. Max
exptime=10 ksec.
Required, even when not
dithering (the Default).
TBD – None at this time.
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† Derived parameters should be visible but non-editable. Derivations are shown in Karakla et al.
(2009) and Beck (2009). †† Formulas related to integration (and exposure) time determination
are revised in Sahu et al. (2010b).
NIRSpec-specific Special Requirements are TBD. Other more general JWST Special
Requirements are found in the Proposal Instructions.
3.3.1 Dithering with the Fixed Slits
Dithering in the FS will be possible, as will dithering between FSs (Tumlinson 2009b).
Some observers will want to dither between the two narrow FSs to recover parts of a
source spectrum lost due to the gap between SCAs (~17."8 on the sky); while others will
want to sample the LSF or PSF by dithering in either the dispersion or spatial direction in
a given slit, or both. Others may simply dither to be able to reduce pixel-to-pixel
variations in the detector, or to correct bad pixels. Observers may combine dithers, to
improve resolution while mitigating detector variations. Up to five spatial dithers will be
possible along a FS, with a range of separations between them. Sub-pixel sampling in
either the spatial or spectral dimension may be performed on top of the spatial dither
positions. Secondary pattern moves may require GS reacquisition. To minimize
mechanism usage, it is recommended that grating and filter changes follow exposures at
all dither positions in the pattern.
Dithers will be specified using the parameters shown in Table 5. From the four choices
presented, the dither pattern can be expanded by the PPS and ground system into a set of
related exposures (and visits as needed).
Table 5 Fixed Slit Dither Pattern Options
Dithers Pattern†
Primary (Spatial) Dither
Number of Positions
Secondary sub-pixel
spectral dither
Secondary sub-pixel
spatial dither
Chip-gap Dither between 2
Narrow slits (A_200_1 and
A_200_2)
Value
Details
Remarks
np= 1 (none),
2, 3, or 5
Choose from menu. Spacing will be
pre-defined. Final values are TBD.
Yes/No
Default positions
Yes/No
Default positions
Yes/No
Large (19."5) dither to
be executed after
primary and secondary
patterns in first slit.
Offered for R=2700 gratings only
(G140H, G235H, G395H), and
only if one or the other narrow slit
is specified for the science aperture.
† Defaults are underlined. Default will be to not dither (np=1, No, No, No).
For high-resolution spectra (G140H, G235H, G395H) obtained through the fixed slits
(except perhaps the A_200_2) and the 1."6 square aperture, there will be a gap in
wavelength coverage of approximately 0.04 to 0.12 µm due to the gap between SCA
devices. For data obtained with either A_200_1 or A_200_2, the gap can be filled with a
dither from one to the other slit, followed by an identical exposure. These two slits are
positioned approximately 19."5 apart for this purpose. This special purpose “chip-gap
covering” dither should be a single choice by the user to request the dither or not, and this
choice will be shown only when using the R = 2700 grating. This type of dither may
require a new target acquisition, depending upon the on-orbit offsetting accuracy of the
telescope.
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3.4 IFU Template
The Integral Field Unit operates over the same wavelength range as the other NIRSpec
modes, with the same disperser (grating & filter) choices. It has a 3" x 3" field of view
dissected into 30 slices of 0."1 width and 3" length each. The slices create a series of
virtual slits that disperse the light from the areas they subtend on an extended source
across the entire detector. The IFU aperture is in the plane of the MSA and is always
open, but is obscured by the MSA magnet arm when the IFU is not in use. The dispersed
light from the IFU virtual slits covers the same area of the detectors as the MSA FOV, so
users might need to carefully plan and verify their IFU observations in the MSA Tool to
avoid overlapping spectra from bright spoilers in the MSA field (for background flux
verification/removal). The IFU template parameters are shown in Table 6.
Depending on the source symmetry, some IFU observations may have few orientation
constraints. The initial planning process should allow the user to specify a wide range of
possible orientations. At Phase II, we expect the orientation to have been selected for
observers as a result of the scheduling process.
IFU observations will generally use the standard MSA TA to accurately center a target in
the FOV. But many IFU observations may not need this centering accuracy and may be
able to use catalog reference star positions and a “Coarse TA” strategy (Section 3.2.2).
Both TA options should be available in the template.
Use cases for the IFU can be found in the report on NIRSpec Observing Scenarios
(Soderblom 2010).
Table 6 The NIRSpec "IFU" Template
Input
Value
Details
Remarks
TA reference stars will
need to be defined.
TA strategy to be used depending on
accuracy needed.
See Table 2
Depends on choice of FS and
science goals.
A. Visit Level Parameters
Standard,
TA Method
Coarse
Accuracy
B. TA Specification
C. Target Specification (Allow multiple Targets per Visit)
Target Name/ID
Specify a target
Choose from menu
D. Instrument Configuration (Allow multiple Dispersers per Target)
Dispersion Element(s)
menu
Specify grating and
filter combination
Choose from menu
E. Exposure Specification (Allow multiple Exposures per Target)
Saturation Possible?
Saturation Flux
Requested Exptime
For details on these parameters and footnotes, see Table 4, the FS Template.
Readout Pattern†
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Input
Value
Details
Remarks
See Table 7
Required, even when not dithering
(which is the default)
Specify via generic
form
TBD – need complete set
Ngrp†
Nint†
Exptime†
Dither Pattern Options
Special Requirements
Yes/No
3.4.1 Dithering with the IFU
Observers may want to dither with the IFU for many of the same reasons they would
dither in other modes – to improve bad pixel subtraction, reduce noise due to pixel-topixel sensitivity variations, and improve spatial and/or spectral resolution. Some may
also wish to map out a large extended object over an area wider than the 3" x 3" FOV of
the IFU. As described in Tumlinson (2009c), this is possible to implement as a dither
because the IFU FOV is small enough relative to the FGS FOV that the required motions
can be executed within a single visit, provided the total size of the pattern is smaller than
2 or 3 arcmin, and that the same guide star is used at all positions.
An IFU Tool (something like the MIRI Mosaic Tool) would be useful for planning
large-scale primary patterns of the IFU FOV to map an area on the sky using just a
handful of parameters. The tool provides interactive tuning of tile spacing and overlap,
orientation, and skew (and exclusion of selected tiles, which may be useful in planning
certain IFU observations). There is one important distinction, though - each “tile” of the
MIRI mosaic tool represents a separate visit. For the IFU, the tool should accommodate a
visit that spans multiple tiles of a mosaic for more efficient planning of observations that
can use the same guide star. APT should be able to insert guide star (GS) acquisitions
and target acquisitions as needed.
IFU background spectra may be acquired by offsetting the instrument to another position
in the field to acquire a suitable background spectrum or spectra for the observation. The
data would then have to be associated so the pipeline can reduce them together. Details
are TBD.
In addition, IFU observers may define a set of slitlet-stepping dithers. A secondary
pattern of slitlet-stepping dithers with the IFU will image light from the same part of the
sky onto different regions of the detector. This technique, designed to mitigate detector
effects, involves stepping over some number of slitlets in the dispersion direction. This
technique may also be important to mitigate spectral contamination caused by spoilers in
failed open MSA shutters. The slitlet number to be chosen is a function of the amount of
overlap the user is willing to accept, and this increases with wavelength for a given step
size. A more detailed explanation is given in the NIRSpec OCD (Böker and Valenti
2010), and Tumlinson (2009c).
Lastly, they may define a set of subpixel dithers in the spatial or spectral dimensions to
help improve sampling. Table 7 lists all parameters for the specification of a complete
IFU dither pattern.
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The complete dither pattern will be executed for a single band (grating & filter pair),
followed by a change of bands, and a repeat of the pattern in reverse. This approach will
conserve grating and filter wheel mechanism moves.
Table 7 IFU Dither Pattern Options
Dither Pattern(s)
Value††
Details
Primary Dither? †
Yes/No
If Yes, use IFU Tool to design dither
tiles. {Nx, Ny, Δx, Δy, and Rotation}
will be specified using the tool.
Secondary Slitlet Dither
N = {1,3,5}
Number of positions along the slitlet
Secondary sub-pixel
spectral dither
Secondary sub-pixel
spatial dither
Remarks
For tiling extended objects
from 3" (an IFU FOV) to 3’
(an FGS FOV).
Choice depends in part on
wavelength of observation.
Yes/No
Default positions
Yes/No
Default positions
† Primary dithers can be specified using a new IFU Tool. †† Defaults are underlined.
3.5 The Bright Object Template
A high priority science goal for NIRSpec is to acquire high S/N spectroscopy of bright
stars to study planetary transits. A large square aperture (1."6 x1."6) was added to make
this possible. Observers will be informed early in the proposal planning process that use
of the special-purpose bright object template is recommended if their target is brighter
than some magnitude (AB = 10, TBD) and mandatory above some brighter limit (AB = 8,
TBD) (Tumlinson, 2009a). The readout pattern for these observations will always default
to NRSRAPID.
A special TA involving a crude offset acquisition followed by a dispersed FS peakup,
described in Beck (2009) was created for this type of observation, which is only made
available through the use of the Bright Object template.
Special sub-arrays are being defined now to prevent saturation of bright targets
(Tumlinson 2009a). Some are full width (2048 pixels), others are half width (1024) but
each of these will cover some reference pixels to aid calibration. A few shorter ones will
be available for use with the PRISM. The plan, as with the regular subarrays available
from the FS template, is to have identically sized subarrays on both SCAs. The starting
column for each will be different because the readout orientation of the two SCAs are
flipped. Detailed implementation is TBD, but is being worked now. The default subarray
will depend on the target brightness and science grating, and should be automatic once
the grating is chosen. Some flexibility of choices will allow the user to cover one or more
interesting spectral features. The same grating and sub-array will be used for the FS
PeakUp during TA.
The template parameters for Bright Object observations are shown in Table 8. Many
details are TBD, since this template has not yet been implemented in the scripts.
For bright object observations, no background spectrum is acquired since none is needed.
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Table 8 Bright Object Template Parameters
Input
Value
Details
Remarks
BO TA will be
the only choice
(not yet
implemented)
Choice will affect
TA Template
parameters, see
Table 2
Initial offset TA strategy to be used
depending on availability of MSA, or
the positioning accuracy needed.
A. Visit Level Parameters
Bright Object (BO) TA
Initial MSA TA
method:
Refinement will be
done using a 7-point FS
PeakUp in the 0."4 slit
and a final slew to the
1.″6 square aperture
Sub-Array
FS PeakUp will use the smallest
possible sub-array based on target
brightness. Same sub-array as science
data. NRSRAPID readout.
Menu
Default sub-array
determined from
target flux (rules
are TBD).
Sub-arrays for large square aperture
bright object observation are being
designed now.
B. Target Specification (Allow multiple Targets per Visit)
Target Name/ID
Specify a target
Choose from menu
C. Instrument Configuration (Allow multiple Dispersers per Target)
Grating and filter pair
Dispersion Element(s)
D. Exposure Specification
Saturation Possible?
Yes/No
Saturation Flux
Requested Exptime
Readout Pattern†
Default is
NRSRAPID,
other patterns
selectable
For details on these parameters and footnotes, see Table 4,
the FS Template
Yes/No
See Table 5
Ngrp†
Nint†
Exptime†
Dither Pattern Options
Special Requirements
Specify via
generic form
Required, even if not dithering (which
is the default, i.e. [np=1, no, no, no]).
TBD – none specific to NIRSpec
known at this time.
3.6 MOS Observations
NIRSpec observers will commonly observe multiple targets distributed over several
nearby MSA fields-of-view. To facilitate planning and executing observations with a
minimum of overheads for this complex mode, a new observing paradigm has been
defined:
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An area of the sky that can be accessed using the same GS defines a NIRSpec field.
NIRSpec Observations will cover one or more fields. An Observation with three such
fields is shown in Figure 1. If the exposure times are short enough, each field can be
observed with a single Visit. Having multiple fields in the same observation will limit
overheads, and can facilitate the uniform processing of associated datasets. Within each
field, a set of objects that are observed simultaneously is called a Target Set. Several
MSA target sets requiring one or more separate target acquisitions can be observed as
part of the same field, provided all can be acquired with the same GS. A small offset
from the previous target set may help to optimally center a subsequent target set and
maximize the number of targets in a given set. Offsets that incur a new TA in the same
visit are allowed provided exposure times are short enough to fit, but offsets that would
involve a second GS acquisition in the same visit are not feasible.
Knowledge of the precise orientation is needed to specify an MSA Configuration - an
accurate map of open or closed MSA shutters. Thus, MSA configurations cannot be
finalized until an initial LRP is made and observers are given a finite orientation range
(or PA range) for their observations. MSA configurations will be planned by an observer
using the MSA Tool in APT. A target set may have several associated MSA
configurations. Individual science targets must fall in the “sweet spot” of an open MSA
shutter, with no other objects in open shutters on the same row that might contaminate the
spectrum. The MSA tool will optimize positioning of shutters to center as many targets
as possible in a set of dithers at a specified orientation. Failed open shutters in the same
detector row as a target must be avoided.
Before introducing the MOS template, it’s useful to consider the structure of a NIRSpec
MOS observation. The observation in Figure 1 has three nearby fields, each with two
target sets. Generally, multiple target sets will be needed for several reasons: as a way to
group sources with similar brightness, for example, to apply appropriate exposure times,
or to improve source centering in shutters. For example, the two target sets in Visit 1 may
be needed to observe high priority targets that cannot be acquired together within the
same MSA Configuration (even if the same TA reference stars are used).
In this Observation, we assume a three-shutter slitlet for each source, with dithers down
the slitlet (positions labeled “Dither1”, “Dither2”, and “Dither3”) in all 4 visits. The two
target sets of Visit 1 can be acquired with the same guide star and their total exposure
time does not exceed the 10 ksec limit, so they fit into a single visit. Also, both target
sets can use a single MSA configuration at all three dither points (labeled C1 and C2).
In the second field, two visits were created because the targets were faint and required
exposure durations that would otherwise have exceeded the limit. Each visit at this
pointing will be preceded by a guide star acquisition (a re-acquisition, in the case of visit
3), followed by a target acquisition with the same set of TA reference stars. Note also that
an MSA reconfiguration (C3 to C4) was needed to complete the dither pattern in each
visit. This is possible if the optical elements before the MSA distort the field irregularly.
Finally, in the third field, though it’s possible to use the same GS and group observations
of both target sets into a single visit, it was preferable to define two sets of TA reference
stars to optimally place the reference stars and science targets into their respective shutter
sweet spots. One possible work-around to this is to define a single super-set of TA ref
stars for a visit where each target set can use a different subset of reference stars. An
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MSA reconfiguration (from C5 to C6) was needed to complete the dither for the first
target set in this visit (Target Set 4), again because of distortion. Because the sources of
Target Set 5 were distributed more favorably, a single configuration was sufficient.
Figure 1 Multiple Fields, Split long–duration Visit
3.6.1 MOS Template
The MOS template fields are shown in Table 9. Many of these parameters will be
determined with the help of the MSA Tool, as described.
Note that for Phase I proposers requesting time from the Time Allocation Committee, it
may be necessary to add template parameters representing the number of target sets
desired, and the number of dither points per target set. These would allow for a
reasonable approximation of total requested time needed since these are the dominant
contributing factors for MSA observations. Nominally, phase I proposers would not be
required to use the MSA Tool to design their program, so proposers would need to
specify the number of target sets a priori to better compute their total requested time.
Once accepted, phase 2 proposers would design actual MSA target sets with the MSA
Tool.
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An optional Confirmation Image can be requested by the observer to verify that the
science sources appear in their planned shutters of the MSA. It could also be used during
data processing to associate individual spectra in the science image with their intended
targets (TBD). This image is taken with the same filter and MSA configuration as for the
science data, and a default exposure time derived from the MSA target fluxes and readout
pattern.
Table 9 Inputs to the NIRSpec MOS Template
Input
Value
Details
Remarks
A. TA Specification. See Table 2 (Reference stars are selected using MSA Tool.)
B. Target Specification
1
(MSA Target Sets and dithers will be designed with MSA Tool. See Part D below.)
Optional direct image (Grating MIRROR in place) following TA
Confirmation
Yes/No
with the MSA. This image is taken with the same filter and MSA
Image?
configuration as for science.
Confirmation
NRSRAPID
Image Readout
NRS
Readout Pattern will default to the same as for the science image.
Pattern
NRSSLOW
Default exposure time will be determined from Readout pattern
Confirmation
and fluxes of MSA Target Set fluxes using an algorithm provided
Image Exposure
Seconds
by the WIT (TBD). User will be able to override default exposure
Time
time by changing Ngrp for the Confirmation image.
C. Instrument Configuration
Dispersion
Element(s)
Specify grating and
filter combination
Choose from menu
1
D. MSA Tool Output (Repeated for each Target Set).
MSA Target Set
Dither Positions
&
MSA
Configurations
Paired one-toone
Defined using the MSA
Tool. See Section 3-7.
Set of targets successfully centered in
shutters.
Optimum positions for the linked set
of dithers.
The MSA Configurations for all
exposures (dithers) related to a
specific Target Set.
E. Exposure Table (Allow multiple Dithers & Dispersers per Target Set)
Saturation Possible?
Saturation Flux
Requested Exptime
Readout Pattern†
Ngrp†
Parameters are derived with the help of the ETC and after the MSA Target Set
members are defined with the MSA Tool. See Table 4 for further details on
these parameters and footnotes.
Nint†
Exptime†
Special Requirements
Yes/No
Specify via generic form
TBD
1
Users should not be required to run MSA Tool to specify Phase I programs. For this reason, parameters
need to be present explicitly in the template to predict reasonable estimates for the total time requested.
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Target set members, dither positions and MSA configurations are not needed for these estimates, but
Number of Target Sets and Number of Dither Positions will factor into the time estimates needed.
Next, we describe how the MSA Tool can provide information for the APT program, and
record results to an MSA Configuration File. The same “Config File” may be utilized in
several exposures of the same target set. The set of related MSA configuration files used
in a linked set of dithers for a given target set should be collected (as files in a database,
e.g.) so that they may be easily referenced and used for re-planning purposes.
Exposures for a given target set might be represented to the user concisely as an
expansion in APT as shown in Figure 2. In this example, the same MSA configuration is
used for two gratings, each with a pattern of dithers up and down the three-shutter slitlet.
As shown, a user would choose the grating, run the MSA Tool to determine target set
members, dithers and associated configs, and then determine the remaining parameters
with ETC assistance. They can also indicate where a TA may be inserted, as shown.
MSA Grating & Sat?1 Peak2 Treq3 RO4 Nint5 Ngrp6 Texp7 TA?8
Dither
Config
Filter
Pattern, Pos9
Flux@λ
Config 1
G140M/
F100LP
✓
200µJy
300 NRS
1
7
297
Config 1
G235M/
F170LP
✓
200µJy
300 NRS
1
7
297
Config 2
G140M/
F100LP
✓
200µJy
300 NRS
1
7
297
Config 2
G235M/
F170LP
✓
200µJy
300 NRS
1
7
297
Y
1, (1-3)
1, (1-3)
Y
2, (1-3)
2, (1-3)
Figure 2 MSA Tool Exposure Expansion for a Target Set
1.
Sat? Indicates which exposures may saturate the detector. 2. Peak flux is the flux below which the user
has indicated that sources should not saturate the detector. 3. Treq is the user-requested exposure time;
4
. RO is the readout pattern determined with the help of the ETC along with 5. Nint (the number of
integrations), 6. Ngrp (the number of groups in an integration), and 7. Texp (the calculated exposure time).
8.
TA? Asks where the user would suggest a TA insertion. 9. Dither Pattern, Pos indicates a dither pattern
id followed by a list of dither positions within the dither pattern.
3.6.2 Dithering with the MOS
For each target set, the observer may specify dithers to sub-sample the LSF or PSF, for
example. Different MSA configurations might be needed at different dither positions.
Note that dithers are linked to target sets. Dithers can occur within a visit, or span several
visits in a field. Larger dithers requiring a new GS would be split into a separate field and
this may require defining a new target set with some of the same members, and possibly
many added ones.
Dithering the MSA to correct for pixel-to-pixel sensitivity variations and dead pixels, and
improve spatial and spectral sampling will be encouraged. Several general observing
strategies discussed in Tumlinson (2009d) and in Section 7.3.5 of the NIRSpec OCD are
summarized below. Dithering parameters must also be passed to the MSA Tool for its
optimization analysis (see Table 10), and final dither positions and MSA configurations
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must be captured into the proposal for use by the downstream proposal processing
system:
2)
3)
4)
5)
a. A variant of the above method to improve the sampling of the LSF is to add a
dither by one shutter in the spectral dimension, an MSA reconfiguration to open
an adjacent slitlet, and another nod up or down the new slitlet.
Slitlet Dithers. A slitlet of Nx1 shutters (default is a 3x1-shutter slitlet) is used for all
targets. Dithers involve nodding the target up or down the slitlet by 1 shutter between
dither points. The MSA configuration will generally remain the same at each dither
point.
Sub-Shutter Dithers (Regan, 2005). Perform a regular pattern (3x3 or 4x4) of subshutter dithers in the spatial and spectral dimensions spanning a full shutter in each
direction. The slitlet in this case would be a 2x1 shutter slitlet to help minimize MSA
reconfigurations, and to provide each target with an associated background spectrum.
This approach is expected to provide better slit loss correction, however many MSA
reconfigurations will be needed. The MSA Tool would determine the position of each
dither and the MSA configuration needed at that dither point. Note that one advantage
of this strategy is that it can be implemented with solely a coarse TA.
Dither to cover the Wavelength Gap. Spectra taken with the R=2700 gratings (and
those with the R=1000 gratings for MSA observations) will have gaps caused by the
gap between the two SCAs. To recover missing portions of the spectra, the MSA will
be moved by about 100 shutters (~20") in the dispersion direction. The positioning of
the MSA would need to be tweaked by the MSA tool to optimize the number of
targets at each dither point.
Customized slitlets and/or Generic Dithers. Slitlets can be individually customized
for each source in a target set. Standardized dithers won’t always be appropriate, so
we must implement a generic method for dithering a target set by small offsets. The
MSA tool would need to determine a linked set of dither positions and MSA
configurations to perform these observations such that the same GS can be used to
keep them within a visit (by limiting the maximum angular separation of all dithers to
~20" (TBD).
3.7 The MSA Tool
This tool was called the NIRSpec Observation Planning Tool (NOPT) in the NIRSpec
OCD (Böker and Valenti 2010). It is now commonly referred to as the “MSA Tool”.
The MSA Tool is used to design MSA configurations for observations of targets chosen
either interactively in APT and written as an “obset” from Aladin, or ingested from an
existing (tsv format) Ascii file as a list of RAs, DECs, and (optionally) fluxes and target
priorities. The user will define their MSA target sets and associated MSA shutter
configurations using the MSA Tool, which will optimize the alignment of the MSA to
obtain the most high-priority sources, given an orientation, a central pointing position and
set of dithers, using the latest shutter quality information. The open/closed shutter
configuration for each dither point will then be written to an MSA Configuration File
(aka “Config file”). Each Exposure of an Observation may have a different Config file,
however some Config files will be re-used at adjacent dither points for small dithers (as
in the slitlet-dither observing strategy discussed above). Each target set may require more
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than one Config file. Since shutter reference info will evolve (possibly every few weeks),
it may be necessary to update Config files associated with a given target set prior to the
execution of a Visit. We need a mechanism to recall all related MSA Config files for
replanning. Even small changes in the pointing and orientation for a given visit will likely
require replanning MSA configurations prior to execution, so these parameters should
perhaps be recorded to the config file.
In Muzerolle, (2009), optimization strategies to avoid stuck MSA shutters were devised
as well as strategies for avoiding spectral overlaps for a variety of slitlet shapes, and for
including prioritized target lists. The effects of adding dithered offsets were also
investigated. The report covers MSA Tool enhancements that could assist PIs in
designing MOS observing programs.
Apart from designing science exposures using the MSA, the MSA Tool will also help
NIRSpec users plan bright object masks (Config files) for their TAs so that bright stars in
the field are blocked from causing detector persistence. Recall that MSA TA can be used
for any NIRSpec instrument mode.
Table 10 Inputs for the MSA Tool
Input
Value
Source List
Overlay Image
Details
Remarks
If no source list is entered, sources are
selected using Aladin from the preimage, or from existing targets in the
proposal.
Specify overlay image
Optional list of
source positions and
priorities.
for Aladin display
Observation Implementation
Slitlet Shape
Slitlet
Sub-Shutter
Dithers
Choices:
NONE
Slitlet
Sub-shutter
Custom
Dither Across
Spectral Gap?
Custom
“Slitlet Dither”
Choose Nx1 or Nx2 and
Choose N={2, 3, 4, or 5}
Specify an acceptable flux calibration
error in order to set the size of the
acceptance zone for a given λ, which
determines if sources are sufficiently
centered in a shutter. Used for MSA
optimization.
Define a 2x1 slitlet. Specify N for an
NxN dither pattern with 1/N spacing.
Slitlets designed using the MSA Tool.
Begin at top of slit and dither by 1
shutter down the entire slit; Repeat at
second column, if Nx2.
“Sub-Shutter”
dither
Step by 1/N shutters across and down
through dither pattern.
“Custom”
dither
User designs a set of linked dither
positions and MSA configs using the
MSA Tool.
Yes/No
{10%, 20%, 30%}
Larger errors imply
larger acceptance
zones.
N = {2,3,or 4}
Determine offsets and
Configs for each
dither point;
implement as
separate exposures
NxN dither pattern,
with 1/N spacing.
Offered only for
R=2700 and R=1000
gratings.
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Input
Value
Details
Remarks
If Yes
Add secondary dither by 18" (TBD) in
dispersion direction. If paired with
another dither, this should be executed
only after completing primary dithers at
first position.
This dither will
require a TA, and
possibly a new GS
acq (and new visit!)
3.7.1 Shutter Quality and MSA Optimization
The MSA Tool will attempt to use the latest shutter quality maps to perform its
optimization. Shutter quality is determined from both ground-based tests prior to launch
and continued on-board monitoring.
1. Stuck-closed/plugged/shorted shutters. The MSA Tool will avoid placing targets
into a shutter that is closed or shorted.
2. Stuck-open shutters. The Tool will attempt to use a stuck open shutter to place a
listed target inside, but must otherwise avoid placing a known source into the
shutter, since its spectrum will contaminate the spectrum of any other source in
the same set of rows affected by that shutter. Affected rows can extend across two
adjacent quadrants for the R=1000 and R=2700 gratings. This may require
knowledge of source fluxes in the entire field, and is expected to be beyond the
capabilities of the tool, at least initially. To protect target spectra then, it might be
preferable to simply have the tool avoid using rows with a stuck open shutter.
There should be extremely few of these, and they are not expected to grow much
in number after commissioning.
Open/closed shutters need to be trackable on a timescale of every few weeks or so,
and a history maintained of when the new specifications were made and the periods
over which they were used. Low-contrast shutters may require similar tracking. We
do not plan to release new versions of APT when a new shutter map is created.
Instead, we plan to perform internal institute checks and/or MSA configuration
releases to PIs to verify that all programs remain largely unaffected.
3. Low-QE regions and/or hot pixels and snowballs on the detector (particularly, we
need to make sure that no target acquisition reference objects lie at the position of
hot or cold pixels).
The MSA Tool should additionally highlight catalog objects within background shutters
so the observer can decide how to handle them. One idea is to indicate these in the “slitlet
view” of the MSA Tool.
Note that stuck open and low contrast shutters can also contaminate IFU spectra. It’s
possible that a future version of the MSA Tool will need to be able to help users plan IFU
observations in crowded fields.
3.8 NIRSpec Special Requirements
No NIRSpec-specific special requirements are defined at this time, but we leave it as a
place-holder for the templates shown in this report in the event that new requirements are
defined.
Check with the JWST SOCCER Database at: http://soccer.stsci.edu/DmsProdAgile/PLMServlet
To verify that this is the current version.
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3.9 Guide Star Availability
GS availability must be part of APT, since an observer will need to ensure that potential
GSs will be available for each observation visit. Guidelines for APT will be defined at a
later time.
3.10 NIRSpec Pre-Imaging (with NIRCam)
The pointing accuracy of the JWST telescope with GSC2 is ~300 mas. This is
insufficient to point and shoot with the MSA. A more precise target acquisition is needed.
Specific shutter configuration necessary to prevent overlapping spectra along the
dispersion direction in the MSA, and precise enough to determine exact shutter
placement will require a-priori knowledge of the source locations. Most observers will
require NIRCam observations to do so.
For a small number of targets, it may be possible to observe all at the same specific
orientation. It may be necessary to break observations into target subsets with different
orientations.
o May require 2 NIRCam fields for comparable coverage with NIRSpec. See
Anderson’s (2010) report on NIRCam dithers.
o Not a requirement, but observer will need to have some IR info, like WFC3
images otherwise, in hand for their Phase II update.
o Can use Special Requirement “EARLY ACQ FOR <visit>” to link observations
o NIRCam Observations will be several months prior to NIRSpec obs, since the
FOR (field of regard) imposes visibility restrictions at most ecliptic latitudes.
The lag in NIRSpec observations must be long enough for the PIs to process
images and create an updated Phase II with defined MOS configs.
4.0 References:
Anderson, J., “Dither Patterns for NIRCam Imaging”, 2010, [JWST-STScI-001738]
Beck, T., “Alternative Strategies for NIRSpec Target Acquisition in FS and IFU Modes”,
2009, [JWST-STScI-001751]
Böker, T., “JWST Near-Infrared Spectrograph (NIRSpec) Target Acquisition
Requirements”, 2008, [ESA-JWST-RQ-5071]
Böker, T., Valenti, J., 2010, “NIRSpec Operations Concept Document”, 2010, [JWSTOPS-001309 Rev B]
Fullerton, A., and the Template Study Group, “Preliminary Definition of Observation
Templates for JWST Science Programs”, 2008, [JWST-STScI-001257]
Karakla, D. and the NIRSpec Exposure Splinter Group, “Defining NIRSpec Exposure
Times and Related Parameters”, 2009, [JWST-STScI-001752]
Kinzel, W., Henry, R., Whitman, R., Donaldson, T., & Valenti, J., “JWST Mosaics” ,
2006, [JWST-STScI-000770]
Kinzel, W. et al. “JWST PPS Operations Concept Document “, 2006, [JWST-STScI000648 Rev A]
Muzerolle, J. “Optimization Strategies for the NIRSpec MSA Planning Tool”, 2009,
[JWST-STScI-001093]
Petro, L., et al., “Science Operations Design Reference Mission, Phase 1 Proposals” ,
2004, [JWST-STScI-000373]
Check with the JWST SOCCER Database at: http://soccer.stsci.edu/DmsProdAgile/PLMServlet
To verify that this is the current version.
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Regan, M., “An Alternative Observing Strategy for NIRSpec and its Effect on NIRSpec
Target Acquisition”, 2005, [JWST-STScI-000674]
Sahu, K., et al., “NIRCam Observing Templates Description”, 2010a, [JWST-STScI002178]
Sahu, K., et al., “Integration Time for a JWST MULTIACCUM Exposure”, 2010b,
[JWST-STScI-002124]
Soderblom, D., et al. “Observing Scenarios for Nirspec”, 2010, [JWST-STScI-002270]
Sparks, W., “Orientation Visualization and Control in JWST APT”, 2009, [JWST-STScI001667]
Stanley, P., Isaacs, J., Kinzel, W., Long, K., Petro, L., “An Operations Concept for JWST
Planning & Scheduling”, 2005, [JWST-STScI-000639]
Tumlinson, J., “NIRSpec Subarrays for Planetary Transits and other Bright Targets”,
2009a, [JWST-STScI-001601]
Tumlinson, J. “NIRSpec Dithering Strategy Part 1: The Fixed Slit Apertures”, 2009b,
[JWST-STScI-001678]
Tumlinson, J., “NIRSpec Dithering Strategy Part 2: The Integral Field Unit (IFU)”,
2009c, [JWST-STScI-001749]
Tumlinson, J., “NIRSpec Dithering Strategy Part 3: The Microshutter Array (MSA)”,
2009d, [JWST-STScI-001769]
Valenti, J. et al. 2006 “Properties of the Software Tool Used to Define Observations with
the NIRSpec Micro-Shutter Array (MSA)”, [JWST-STScI-000758]
Check with the JWST SOCCER Database at: http://soccer.stsci.edu/DmsProdAgile/PLMServlet
To verify that this is the current version.
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APPENDIX: Acronyms
ACQ
APT
AU
BO
Dec
DMS
FOV
FS
GS
IFU
JWST
LRP
MOS
MSA
NIRCam
NIRSpec
OCD
PA
PM
PPS
RA
SCA
TA
TAC
Acquisition
Astronomer’s Proposal Tools
Astronomical Unit
Bright Object
Declination
Data Management System
Field of View
Fixed Slits
Guide Star
Integral Field Unit
James Webb Space Telescope
Long Range Plan
Multi-Object Spectrograph
Microshutter Array
Near Infrared Camera
Near Infrared Spectrometer
Operations Concept Document
Position Angle
Proper Motion
Proposal Planning System
Right Ascension
Sensor Chip Assembly
Target Acquisition
Telescope Allocation Committee
Operated by the Association of Universities for Research in Astronomy, Inc., for the National
Aeronautics and Space Administration under Contract NAS5-03127
Check with the JWST SOCCER Database at: http://soccer.stsci.edu/DmsProdAgile/PLMServlet
To verify that this is the current version.