TECHNICAL
REPORT
Title: Defining NIRSpec Exposure Times
and Related Parameters
Doc #:
JWST-STScI-001752, SM-12
Date:
May 4, 2009
-
Rev:
Authors: Diane Karakla
Phone: 410and the NIRSpec Exposure 338-4947
Splinter Group1
Release Date: 8 July 2009
1.0
Abstract
This technical report describes the scheme we propose for the JWST proposal definition
templates for planning observations in each of the three NIRSpec observing modes: the
Fixed Slits (FS), the Integral Field Unit (IFU), and the Multi-Object Spectrometer (MOS)
using the micro-shutter array (MSA). In particular, we explain how observers will be
able to specify a small set of parameters from which more detailed exposure level inputs
needed for the scripts can be derived.
2.0
Introduction
The Mission Operations Concept Document (MOCD) defines important parameters that
specify the exposure time for JWST MULTIACCUM data acquisition. So that external
users do not have to be bogged down with the (often confusing) terminology, Beck et al.
(2008) proposed a method to specify exposure times in MULTIACCUM readouts using
sets of user inputs that are (hopefully) more understandable than those in the MOCD.
These inputs are the: 1) user-selected detector sub-array, 2) user-selected “bright object”
versus “faint object” observing and 3) user-requested on-source exposure time. However,
the “bright object” and “faint object” observing constraints are nebulously defined, and
Beck et al. (2008) noted that these should be removed with better definition of the
exposure time strategy. Here we further define and clarify these exposure time
parameters for NIRSpec science.
1
Diane Karakla, Tracy Beck, James Muzerolle, Bill Sparks, Jason Tumlinson
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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3.0
Exposure Time Parameters
We propose that the user first selects the subarray used for science (in the case of FS
mode only – IFU and MOS observations are always full-frame) and defines their
requested on-source exposure time.
The object brightness at the observed wavelengths and in the chosen subarray (FS only)
will directly influence the maximum duration of an integration to avoid saturation, since
the detector resets occur only at the start of an up-the-ramp integration. Depending on the
saturation limit, an exposure may need to be broken into several integrations to achieve
the desired total exposure time.
The default readout pattern, NIRSRAPID, with 1 frame per group defines a particular
cadence, and consequently, the minimum possible integration time. The three available
NIRSpec readout modes are shown in Table 1. NIRSpec exposures will always default to
a readout mode of NRSRAPID, for 10.6 second group times that allow for maximal
rejection of cosmic ray events (TBR, if data volume is an issue). Table 1 was extracted
from the Operations Concept Document (OCD) and updated with the more recent value
of the maximum integration times possible in each mode. The readout mode should be
user-selectable to NRS, but we will never use the NRSSLOW readout mode.
Table 1 NIRSpec Readout Patterns (updated from the OCD).
Readout
Pattern
Integration Time (s)
Usage
Maximum Minimum
NRSRAPID
10600
10.6
NRS
10600
42.4
NRSSLOW
10600
212.0
Default pattern, preferable for most
sources
for fainter sources or if Data Volume
constraints exceeded
Never used outside of target acq, and
not selectable by user. Reserved for
very faint sources or to be used if
data volume constraints exceeded
Maximum
ngroups
1000
250
50
Minimum timing is for a single group full-frame readout. For NRSRAPID, there are 1 frames per group
(nframe=1), while for NRS, 4 frames are coadded (nframe=4). NRSSLOW is included for completeness,
but will likely never be used, except for faint object target acquisition.
Table 2 outlines the important parameters for defining exposure times with NIRSpec.
This has been updated from Beck et al. (2008), and a few parameters have been added or
corrected (such as tint). In addition to the familiar terms used in Table 5-2 of the MOCD
and discussed in Beck et al. (2008), we also present new values like the elapsed time, and
detector observing efficiency. Every integration is preceded by a detector reset, which
adds an overhead time equivalent to 1 group time. Furthermore, NIRSpec observations
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(and those for nearly all JWST instruments) will have a minimum integration time that is
equivalent to ngroups=2. The final integration will be a subtraction of the first read from
the second, for a “correlated double sample” or CDS.
If saturation will be an issue for science targets, we propose that the user will click a
radio button to indicate this and input a target flux value (or average target flux value)
instead of the “bright object” / “faint object” selection previously defined by Beck et al.
(2008). Ideally, the user can expect targets with fluxes below this limit will not
saturate. This is an important distinction, when observing >100 targets in MOS mode, it
will not be feasible to define the fluxes for all sources. But users must define some flux
level below which they wish to avoid saturation. This flux value will be used to define
the “tmax” value, which is the maximum amount of time that can be spent on an
integration to avoid saturation (see below, and Beck et al. (2008)). We hope that the flux
information input by the user can either be in a type-able field format, or be linked to
outputs from the ETC to include information on the spectral shape of the target. The
methodology that we use here may also be applicable for defining exposure times with
the other instruments.
If saturation will not be an issue, the user needs only to define the requested exposure
time and the detector subarray. In this case, it is not necessary to define a target flux for
deriving tmax . It is assumed that tmax = tint = texp. The hard limit for tmax in this faint
object target regime is ~10,000 sec, constrained by the time necessary for the observatory
to repoint the high-gain antenna. Regan and Stockman (2001) show that an optimal
maximum integration for faint object exposures is ~3000 - 4000 sec, which balances
target signal-to-noise versus read noise, dark current, and cosmic ray rejection. In the
future, NIRSpec may adopt a tmax value of 4000 sec for faint object spectroscopy
following this recommendation (TBR). For now, tmax is constrained by the 10,000 sec
for repointing the antenna (texp(max) = 10000 sec).
Table 2 Updated NIRSpec Exposure Parameters
Value
Descripion
Defined by:
texp
(requested)
User input
The maximum requested exposure
time for a given exposure texp (max)
is 10,000 sec.
SUBARRAY
Target Flux
User input
User input (or, ideally, an ETC input)
READOUT
PATTERN1
The READOUT PATTERN will default to NRSRAPID.
(Should be user-selectable to NRS; This is TBD see
section 3.0).
Used to determine if saturation on a
source will be an issue, and further
used to constrain the value for tmax.
The READOUT PATTERN will depend
on the user inputs, but will be
constrained by the pre-defined
READOUT PATTERNs that exist for
NIRSpec – namely NRSRAPID and
NRS.
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Value
Descripion
Defined by:
tgroup
The group time.
tmax
The maximum allowed time for a given integration.
tint
tint is the integration time within a single integration,
this is the important time for scientific analysis. Note
that tint will likely not equal tmax if sources are bright
and saturation is an issue. This is because all
integrations are defined to have the first read serve
as a ‘zero’ frame which will be subtracted off of the
final integration (the exception is perhaps the
brightest sources in the fixed slits - TBD).
nint is the number of integrations within a single
exposure and will be defined as follows for bright
object exposures. For faint object observations, nint
will equal 1 (texp< or = tmax).
Derived number of groups in the integration.
nint
ngroups
Defined by the READOUT PATTERN
and SUBARRAY
If saturation is an issue, set by the
target flux information, and/or
instrument configuration (or
instrument properties, background
level, TBD). tmax values will likely be
quantized, based on instrument
sensitivity information.
= texp(requested)
for faint objects
= tgroup x [ [tmax/tgroup] -1 ]
for bright objects
=
=
texp (delivered)
telapsed
e
Delivered exposure time, which may differ from the
user request.
The elapsed time estimate, or the amount of clock
time necessary to acquire the data needed for
scientific analysis.
Detector Observing efficiency estimate – the fraction
of time integrating and acquiring scientifically useful
science data versus the elapsed clock time. Note
that this efficiency is associated with detector
parameters only, and does not include overheads
associated with offsetting, instrument configuration
changes, etc. For a minimum integration time,
ngroups=2 and the observing efficiency is only
0.33.
texp/tint
*
*
(texp(requested)/nint)/tgroup
= nint x tgroup x ngroups
= nint x tgroup x [ngroups+1]
= [ngroups-1] / [ngroups+1]
* - A bracket with closures at the top is the mathematical symbol for rounding up, encompassed in the
“ceiling” function in most common programming languages. The choice of rounding up instead of down
here was made to preserve the user requested exposure time. In practice, the rounding direction is TBD and
could be changed based on the difference between the user-requested and the delivered exposure time.
4.0
Saturation Considerations
If saturation on a source within the requested exposure time will be an issue for a science
target of interest, the maximum exposure time on this object, tmax, will depend primarily
on the flux of the target, the mode of the observation, and the detector subarray used.
The integration time spent on a source is quantized in the MULTIACCUM readout
patterns, so corresponding values for the longest possible integration time without
saturating on the source and the saturation fluxes of the targets observed will also be
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quantized. Table 3 presents saturation flux values for the different spectral resolutions
available with NIRSpec in the MOS and 0.”2 Fixed Slit observing modes. The numbers
presented in this table are in AB magnitudes (first number) and milli-Janskys (second
number).
Table 3 : NIRSpec Saturation Magnitudes/ Fluxes for a Full Frame Readout of a Single
CDS Observation (MOS and FS Modes – 0.”2 wide slits) which corresponds to tmax = 21.2
seconds.
Spectral
Resolution
Band 1
@ 1.3µ m
Band 2
@ 2.4µ m
Band 3
@ 3.7µ m
R=2700
AB Mag
9.9
mJy
395.0
AB Mag
10.2
mJy
315.0
AB Mag
10.2
mJy
317.0
R=1000
11.3
113.0
11.5
93.0
11.5
91.0
R=100
AB Mag
14.8
mJy
4.3
The next three figures show the Saturation Flux value (the target flux that corresponds to
the detector saturation level) plotted versus the number of groups in a MULTIACCUM
integration. The plots are presented for full-frame readout only, and the three panels
correspond to R=100, R=1000 and R=2700 spectroscopy, respectively. Plotted in color
in each figure are the saturation fluxes associated with each “Band” or wavelength range
available with NIRSpec. The value for tmax is merely the number of groups multiplied by
the group time, but we plot ngroups here because this is the value that is quantized. Note
that ngroups = 2 is the minimum defined MULTIACCUM pattern (correlated double
sample), and the plotted fluxes at ngroups = 2 correspond to the values presented above
in Table 3. As expected, bright sources can only be observed in very short integrations
with poor efficiency. As ngroups increases to longer values for tmax, the saturation flux
correspondingly decreases.
The NIRSpec saturation fluxes and detector sensitivities presented here are based on the
model calculations presented by Jakobsen (2007), and these must be updated for the true
performance of the instrument. Once instrument performance is better established, tables
(or equations) of saturation fluxes versus tmax can be provided for all NIRSpec spectral
resolutions, science wavelength ranges, and detector subarrays. This will provide easy
translation between user input target fluxes and the corresponding values for tmax.
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Figure 1: The target
saturation flux plotted
versus ngroups for
R=100 spectroscopy in
MOS and 0.”2 FS
modes using fullframe readout.
Figure 2: The target
saturation flux plotted
versus ngroups for
R=1000 spectroscopy
in MOS and 0.”2 FS
modes using fullframe readout.
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Figure 3: The target
saturation flux plotted
versus ngroups for
R=2700 spectroscopy
in MOS and 0.”2 FS
modes using fullframe readout.
5.0
Use of Exposure Parameters in the Observation Planning Templates
More specific information about how the various exposure time parameters could be
handled in the templates is given in this section. Each of the NIRSpec observing modes,
FS, IFU, and MOS observations with the MSA, is addressed separately below, following
a description of the plan for specifying related target information. User inputs are
identified by bold text throughout the remaining sections.
5.1
Target Info (for Pointing)
Target info will be provided in a separate section of each of the NIRSpec templates. For
all NIRSpec apertures, “Target” refers to the pointing of the center of the aperture.
However, for the MSA, the center is located between the 4 quads, in an area that is not
exposed to the sky.
A few of the parameters in the target info section of the observation planning templates
need more clarification as they may depend on the NIRSpec mode:
• RA and Dec are specified here for each target. These are required for observations in
any NIRSpec mode.
• Proper motion, epoch, etc. is specified for IFU and Fixed Slit mode observations
only.
• Target Flux is specified here and is required for Fixed Slit and IFU modes only. It is
potentially useful for defining target acquisitions for observations in these instrument
modes, but is also here for informational purposes; signal-to-noise calculations,
saturation avoidance, and scheduling. User may specify J, H, K, or M flux. Standard
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flux units will be micro-Janskys (mJy), but conversion tools will hopefully be
available when inserting flux values. The exact flux specification is under study and
is TBD. As mentioned in Section 3, it is assumed that in cases where danger of
saturation exists at any wavelength that will be observed, a link to the ETC output or
the user-specified flux information will be required.
Since visits will have a given pointing, and a given set of guide stars, a pointing requiring
a different set of guide stars will necessitate defining a new visit. Visit durations of up to
1 day will be possible. However, only one instrument mode may be used per visit (e.g.
science with the IFU and MSA must be in different observations).
5.2
Fixed Slits/IFU Template
For the Fixed Slits only, the user will have the opportunity to specify a choice of several
possible subarrays. These are designed primarily for bright objects which might
otherwise saturate in the first few reads of the fastest mode. Subarray will be chosen
from a pull-down menu. Possible choices and a more detailed description of each are
found in Tumlinson (2008). One possible choice is “ALL SLITS”. It covers all of the
regions where the slits are imaged onto the detector. This selection will be required for
dithering between the two 0.”2 Fixed Slits. Once selected, the chosen subarray will apply
to all exposures in the visit.
To further define exposures in a visit, the user will complete a table for selected
dispersers (Grating/Filter combinations). The visit template for observations in the IFU or
Fixed Slits will contain a table like that in Figure 4. The user will enter several values in
the template, and other values will be derived from those.
Grating/Filter
Pull-down menu to
select Grating &
filter combo #1
Grating/Filter 2
Grating/Filter 3
…
Sat?

Peak Flux@λ
or ETC result
User input or
ETC result, plus
option to use
SAME SET FOR
ALL configs
treq
User
input
Readout
Pattern
Derived
value, but
selectable to
NRS.
nint
Derived
value.
ngroups
Derived
value.


Figure 4. Schematic Visit Template for IFU and Fixed Slits
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texp
Derived
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1.
First, the user will select the grating and filter pair from a pull-down menu.
Additional grating and filter combinations will constitute separate rows of the
table.
2.
The user will then enter a requested exposure time, texp(requested) , referred to as
“treq” in the tables below, for each disperser.
3. For each grating combination used in the visit, the user will be able to click a radio
button () to indicate a target could saturate in default NIRSRAPID readout mode in
the grating. The default assumption is that saturation will not be a problem.
A. If the radio button in step 3 is depressed by the user…
An additional pull-down menu will allow the user to choose how the peak
flux will be specified:
i. User input Saturation Flux, or
ii. ETC Info.
If “i” is selected, then the user will enter the brightest flux value in the
selected grating. Additionally, the user should have the option of applying
the specified flux to all exposures (all gratings) in the visit. APT will then
compute tmax, the maximum integration time allowed, or to avoid
saturation, whichever is smaller. The relationships for computing tmax are
described in Sections 3 and 4.
If “ii” is selected, then the user will enter an ETC reference number. The
use of the ETC info is optional and especially desirable if multiple
gratings are used. Additionally, the user should have the option of
applying the ETC results to all other exposures (all gratings) in the visit.
The maximum integration time allowed or to avoid saturation, tmax, will be
calculated from flux values provided by the ETC using the flux to tmax
relations presented in Section 4 (TBR).
B. If the Radio Button in step 3 is NOT depressed…
The default readout mode, NRSRAPID, will be used, excluding any data
volume concerns. For faint sources in no danger of saturating the detectors, a
single integration per exposure is preferable (nint=1, and texp=tint).
4. At this point, the user–defined parameters specified above can be used to derive the
remaining operational parameters listed below and in Table 2 that are used
downstream by the commanding scripts to define an observation. These parameters
are listed in the sequence they will be derived.
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•
•
•
•
•
5.3
The Readout Pattern is chosen for the user. Each Readout Pattern has a specific
cadence equivalent to the minimum integration time listed in Table 1. The
Readout Pattern selected will depend on the chosen subarray and the requested
exposure time, texp (requested). If there are no data volume violations (by rules
TBD), then NRSRAPID mode will be used. Fainter sources requiring longer
exposure times may necessitate the use of NRS. Since data volume is proportional
to texp (requested) /tgroup where 1/tgroup is the cadence, if the ratio violates the data
volume constraint for NRSRAPID, then the NRS mode will be used. (The
expectation is that NRSSLOW will never need to be used.)
Next, tint , the time for a single integration, can be calculated. For most
observations, this will likely equal the requested exposure time, texp (requested).
For bright sources, it will be computed from tmax as in Table 2.
nint, the number of integrations in an exposure, is currently derived by rounding
up the ratio texp (requested) /tint. Rounding direction (up or down) may be
changed later and is TBD.
ngroups is currently derived by rounding up the quantity [(texp
(requested)/nint)/tgroup] where tgroup is defined by the Readout Pattern and is
equivalent to the minimum integration time listed in Table 1. Rounding direction
(up or down) may be changed later and is TBD.
texp(delivered) is the derived or delivered exposure time and is the product nint x
ngroups x tgroup . (The upper limit for the exposure time is also 10600 seconds,
set by the longest time that can be spent integrating without repointing the highgain antenna.)
MOS Template
For planning MSA observations, users will have a tool to help design the MSA
configuration needed for multi-object spectroscopy of a set of ~100 (or more)
sources in the same field. The tool will take into account the geometric
transformation between objects on the sky and locations in the MSA and will
have some capability for helping users to optimize the orientation and pointing of
the MSA apertures while avoiding stuck shutters and overlapping spectra. We
envision that the MSA Planning Tool will be available in APT for users designing
their NIRSpec proposals, and will be part of both the Phase 1 and Phase 2 tool
sets. Several configurations may be needed to cover all sources of interest, due to
the difficulty in getting all sources into the “sweet spot” of a working shutter in a
single pointing. Also, users may want to dither their observations, which might
require different MSA configurations due to the effect of geometric distortions
across the MSA apertures.
The visit template for specifying NIRSpec MSA observations will follow the
scheme outlined in Figure 5.
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MSA config
Config file 1
Config file 2
Config file 3
…
Grating
&Filter
Pull-down menu
to select grating
& filter pairs,
plus option to
use SAME SET
FOR ALL configs
Grating+Filter
Pair 1
Grating+Filter
Pair 2
Grating+Filter
Pair 3
Grating+Filter
Pair 1
Sat
?

Peak Flux@λ
or ETC result
User input or ETC
result, plus option
to use SAME SET
FOR ALL configs
treq
User
input
for
each
grating
Readout
Pattern
Derived
value.
nint
Derived
value.
ngroups
Derived
value.


Grating+Filter
Pair 2
Grating+Filter
Pair 3
Figure 5. Schematic Template for MOS Observations with MSA Configurations
and Grating and Filter choices.
The sequence of steps for completing the template are as follows:
1. The user will enter the name of an MSA Configuration file created with the
MSA Planning Tool. This is a map of open and closed shutters for the
observation. Several configuration files may be used to observe different sets
of targets in the same or nearby fields.
2. The user will select a grating/filter pair from a pull-down menu. One or
more combinations can be selected for each MSA configuration. All gratings
will be observed at each dither location with the same configuration file
before moving to the next dither point. This is to help preserve MSA lifetime
by limiting the total number of MSA moves required.
3. Users will press a radio button on the left to indicate that one or more targets
could saturate in default NIRSRAPID mode. If the button is depressed, then
ETC results at various observational wavelengths will be used to inform the
choice of Readout Pattern and other derived parameters for all MSA
configurations. (See the list of derived parameters in the IFU and Fixed Slit
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texp
Derive
d
value.
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template discussion above in Section 5.2, point 4. for more information about
the radio button and tmax which is directly applicable here).
4. The user will then enter a requested exposure time, texp(requested), for each
grating and filter combination using the same MSA configuration.
5. Readout Pattern , nint, ngroups, and texp(delivered) are derived as
described in IFU and Fixed Slits template (Section 5.2) and in Table 2.
6.0
Summary
We have identified a simplified strategy for defining exposure times in the NIRSpec
observation planning templates using parameters somewhat more familiar to observers.
Study has shown that the NIRSRAPID mode will likely be optimal for most NIRSpec
observations, so we were able to use this knowledge to design a strategy that would hide
some of the detailed parameter choices observers would otherwise have to make. The
calculations of parameters needed by the downstream observation planning system will
be computed from more familiar user-specified parameters like requested exposure time
and saturation flux. Work is ongoing to better define visit parameters described here in
relation to dithers and the MSA Planning Tool (Muzerolle et al., 2009).
7.0
References
Beck, T., and the Integration Time Splinter Group, “Preliminary Definition of Exposure
Times in JWST Templates”, 2008-06-02 [JWST-STScI-001439]
Fullerton, A., “Preliminary Definition of Observation Templates for JWST Science
Programs”, 2008-02-22 [JWST-STScI-001257_RevA]
Jacobsen, P. “Error Budget for the NIRSpec Target Acquisition”, 2005-11-22 [JWSTREF-005938]
Jacobsen, P. “Observing Eclipsing Exoplanets with NIRSpec”, 2007-09-10, private
communication
Muzerolle, J., 2009, in preparation.
Long, K., “Mission Operations Concept Document (MOCD)”, 2008-05-15 [JWSTSTScI-000021_RevB]
Regan, M. W., et al., “NIRspec Operations Concept Document”, 2004-02-20 [JWSTSTScI-000403 [A]]
Tumlinson, J., “NIRSpec Subarrays for Planetary Transits and other Bright Targets”,
2008-10-27 [JWST-STScI-001601]
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