TECHNICAL
REPORT
Title: NIRSpec Autocals
Authors: T. Keyes, T.
Beck, J. Tumlinson and
NIRSpec CalWG
Doc #:
Date:
Rev:
Phone: 410 –
338 - 4975
JWST-STScI-002484, SM-12
29 June 2011
-
Release Date: 19 August 2011
1.0 Abstract
This document describes the current operational status of internal wavelength and flat
field calibration capabilities for NIRSpec and their implication for meeting required
objectives in order to properly calibrate NIRSpec spectroscopy. We present sciencebased requirements for automatic wavelength and flat calibration and discuss
philosophies for implementing automatic calibrations (autocals) to be acquired within the
same visit as science observations. We provide operational details for the
implementation of autocals and a summary of the current operational planning and open
issues.
2.0 Introduction
At the present time, it is not possible in the NIRSpec templates to do a calibration
exposure at the same time or in the same visit as external science observations. Lamp flat
field and wavelength calibration exposures are included within the “Engineering
Templates” for the NIRSpec internal lamp calibrations. As presently defined, the lamp
calibrations would likely be acquired in their own standalone visit, separate from science
exposures. Hence, it is presently not possible to acquire a lamp exposure at the same
instrument configuration used by the science without moving components in-between,
because the NIRSpec instrument mechanisms are moved to a default position at the end
of each science visit.
In this document, we discuss philosophies for implementing automatic calibrations to be
acquired within the same visit as science observations. For proper calibration of
NIRSpec spectroscopy – particularly multi-object spectroscopy using the MSA shutters –
it will be crucial to have these “autocal” functionalities implemented, particularly in the
early cycles of the JWST mission when the NIRSpec MSA “calibration model” is still
being constructed. We provide draft operational requirements and list open questions,
some of which will be resolved by results from the NIRSpec instrument flight model
(FM) cryo testing campaigns. Some initial results have been derived from NIRSpec
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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Cycle 1 calibration, which took place in February 2011 and concentrated on the FS and
IFU observing modes. However, the "FM Cycle 2" campaign, which will focus on the
MSA calibration, is still pending. Full analysis of the data from both campaigns will be
necessary to resolve many of the outstanding issues
3.0 NIRSpec Auto Wavecals
3.1 Wavelength Calibration Exposures:
NIRSpec wavelength calibration (wavecal) exposures can be acquired using two different
Calibration Assembly (CAA) configurations:
a.) narrow-band filter, Fabry-Perot (FP) filter, continuum lamp exposure crosscorrelation location of several (of order 5) broad features to establish dispersion
curve and wavelength zero-point offset
b.) Erbium line absorption filter lamp exposure, measurement of several absorption
lines to precisely set zero-point offset
3.2 Wavecal Exposure Overheads:
Exposure Overheads: FP lamp exposures may not exceed 100 seconds to avoid
significant temperature-related transmission characteristic changes of the filters; in order
to maintain the temperature stability of the FP filters, following any lamp exposure the
lamp must be off for 1000 sec prior to again turning on the lamp (DRD-OPS-02, 2010;
JWST-REF-007587, 2011)). Overheads will be required for mechanism movements to
configure the filter for taking wavecal, to turn the lamp on, and finally, the lamp exposure
itself (< 100sec), then, if needed, a move back to the science configuration (filter) before
continuing. An initial crude estimate is a total of 4-6 min per wavecal exposure.
3.3 Wavecal-related Accuracy Requirements:
After calibration, the wavelength scale of NIRSpec spectra shall be determined with an
accuracy of better than 1/8 of a spectral resolution element (based upon NSFR-15). In
order to meet this requirement using the Fabry-Perot filter wavecals, the zero-point
wavelength calibration accuracy for analysis of 5 peaks in the wavecal data will need to
be better than 5% of a resolution element (Ferruit, 2005).
Hence, we are operating under the assumption that taking a Fabry-Perot wavecal will
accurately establish the wavelength zero-point for a given exposure. The Erbium line
absorption filter lamp exposure will provide wavelength calibration zero-point accuracy
that is better than the Fabry-Perot filter data (amount to be determined in FM testing).
NIRSpec grating wheel native re-positioning accuracy is ~2 pixels. Initial indications
from FM cycle 1 are that the grating wheel assembly (GWA) sensor may be sufficient to
calibrate NIRSpec in orbit, but further verification is necessary, and STScI needs to
review position sensor reading calibration information acquired in all FM testing. At
present no calibration of the GWA sensor is included in commissioning plans, but will be
in cycle 1 calibration observations. In principle we get sensor readout in telemetry every
time we move the grating mechanism. Furthermore, the current plan and templates do
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not provide for wavelength accuracy better than that provided by the wheel sensor and
the normal calibrations, even if these meet the 1/8 resel requirement.
3.4
3.5
Key Issues for NIRSpec Wavecals:
1. Users will want to eke out as much kinematic accuracy from NIRSpec data as
possible. Right now, the wavelength zero-point calibration accuracy is assumed
to be derived from the reported position of the grating wheel position sensor. If
an observation is taken, and the wavelength calibration based on the NIRSpec
grating wheel position sensor is off in comparison to prior knowledge of a target,
how do we verify the true calibration of the data? If the position sensor zeropoint measurement does seem to have problems, how do we verify that the
calibration is accurate in prior or subsequent science exposures?
2. A manual capability for taking internal wavelength calibration exposures will be
available for calibration purposes, but at present is not available to the GO. At the
present time, it is not possible in the engineering templates to take a wavelength
calibration immediately following an external science exposure, without moving
any instrument mechanisms. If we provide for automatic wavecal exposures in
defined circumstances, we will be able to directly evaluate and monitor the
accuracy of the grating wheel position sensor in a routine manner and will be
prepared to deal with any problems associated with the sensor.
3. In the current template structure the user (GO or instrument scientist) can not
insert manual wavelength calibration exposures in a science visit. The manual
wavecal exposures must be in a separate template and visit. For example, we can
not presently specify an external observation of a radial velocity standard target
followed immediately by an internal wavecal with the same configuration. The
grating wheel / filter configuration will likely be reset at the end of any science
visit, so currently a mechanism movement must intervene between a science
exposure and any internal wavecal exposure and would compromise wavelength
calibration accuracy.
Operational Philosophies for Auto Wavecals
Option 0: no auto wavecal exposures are performed (the current plan).
Option 1: perform auto wavecal exposure for each configuration (we define a unique
configuration as a combination of MSA shutters AND grating position),
that is, perform before the MSA + grating configuration is changed (user
selectable).
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NOTE: If a confirmation image of a target set is requested, then the grating
wheel will be moved again and this also implies a new wavecal is required
(i.e., a wavecal must be taken after the grating wheel is in its final science
configuration with a grating in place).
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If this calibration is not performed, then users are depending only on the
NIRSpec instrument model for most MSA configurations.
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This procedure builds up a library of results for comparison with the model
for verification and expansion, and will serve to provide information to verify
the grating position sensor.
4.0 NIRSpec Auto Flats
NIRSpec flat field lamp exposures will be acquired with continuum flux lamps in the
CAA. As with wavecals, in the current template structure the user (GO or instrument
scientist) can not insert manual flat field lamp exposures within a science visit. The
manual flat exposures must be in a separate template and visit. The grating wheel / filter
configuration will be reset at the end of any science visit, so a mechanism movement
must intervene between a science exposure and any manual flat exposure.
Auto flats provide an efficient means of obtaining MSA shutter aperture throughput.
Simulated lamp exposures show that contemporaneous flat lamp exposures also facilitate
location of the spectrum extraction aperture.
4.1
Operational Philosophies for Auto Flats
Option 0: no auto flat exposures are performed (the current plan).
Option 1 - auto flat through each unique MSA plus grating configuration to verify
instrument model (potentially with user-selectable options to designate signal-to-noise in
the flats, which map to the requested flat field exposure time). The flat exposure follows
the science exposure.
4.2
Auto Flat Open Items:
1. What are the exposure times that are used for auto flats, and should there be just a
single exposure time, or a few possibilities to correspond to flat fielding signal-tonoise? (TBD)
2. Overheads for flat exposures may be an important consideration in the decision to
use them for particular science goals. Flats have been variously estimated to
require as little as 100 seconds and as much as 900-1200 seconds exposure time.
Accurate values for S/N as function of exposure time will be determined in FM
testing.
5.0 Autocals: Notes on Implementation
NIRSpec is required to deliver wavelength accuracy to better than 1/8 of a spectral
resolution element (NSFR-15), or approximately 15 km s-1 for spectra taken with the R =
2700 gratings. The NIRSpec design implements two possible methods for achieving this
level of accuracy. First, the grating wheel assembly contains a position sensor that reports
the GWA position in telemetry, so that the wavelength solution can be corrected for small
off-nominal offsets in a given exposure. Second, the NIRSpec Calibration Assembly
(CAA) contains lamps with absorption and emission line spectra for measuring the
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absolute wavelengths (REF) and dispersion solution (LINE1-4) on the detector.
By design the GWA position sensor should provide adequate information to meet the 1/8
resel requirement. There may be non-repeatability in the GWA position sensor: it may
report a value different enough from the actual GWA position to fail the requirement, and
moreover this error could vary erratically from exposure to exposure. The true
performance of the GWA position sensor will not be known before the end of full FM
testing in late 2011, and in any case the GWA position sensor could perform well in
ground test and poorly in flight.
Crucially, the current flight scripts do not have the capability either to fully test the GWA
position sensor performance in flight or to verify the wavelength solution for a given
observation. As the scripts stand now, lamp exposures are possible only in their own
template (e.g. visit). It is not possible to obtain a lamp exposure for a given science
exposure, without GWA motions in between. As a result, if the GWA behaves erratically
there will be no way to know it for any particular observation. Furthermore, there will be
no way to obtain any wavelength solution that is better than the GWA can provide, even
if the sensor performs well.
We propose to implement “auto wavecals” as lamp exposures which occur just after or
before the corresponding science exposure without an intervening GWA motion. This
capability will allow for in-flight checks on the GWA position sensor and will provide
the best possible wavelength calibration to our users. There is no better wavelength
solution than one taken adjoining science exposures with no changes to the instrument
configuration in between.
The auto flats are required to verify the “calibration model” of NIRSpec MOS (MSA)
mode by obtaining data on the response to each shutter / pixel combination for the actual
pattern of shutters used by a science observation. Since this response curve depends on
both shutter and wavelength, we require flat field exposures in the correct flat lamp for
each unique combination of grating and MSA configuration. These exposures should
follow their science exposure to mitigate detector persistence effects. These flat field
exposures will be uniform and deterministic and depend only on the YES/NO choice in
the user template.
Notes on the implementation of these auto-calibration exposures are provided in the rest
of this section.
5.1
Auto Wavecals
5.1.1 Operational details:
Auto wavecal lamp exposures depend on the band of the grating used in the
corresponding science exposure. These exposure parameters are valid for FS, IFU, and
MSA spectroscopy; that is, the exposures do not depend on operating mode. All
exposures use NRSRAPID, NINT = 1, and full-frame readout. The Erbium reference
lamp (REF) exhibits lines only over 1.3 - 1.7 µm, or Band 1. Thus REF lamp exposures
will automatically follow the corresponding LINE1 exposure if and only if the Band I
gratings G140M or G140H are specified by the science template.
This implementation adds only one parameter to the science templates for FS, IFU, and
MSA spectroscopy: AUTOWAVECAL = [YES, NO]. The default behavior will be
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decided as a policy matter but we recommend defaulting to YES for the early portion of
the mission, to build a database of calibration information and ensure archive quality.
Whenever the GWA has been moved to a new position, these auto wavecals should
happen after the corresponding science exposure. To minimize impact from persistence
they should also occur after any auto flats that have been selected for that science
exposure. The optional confirmation image allowed for MSA spectroscopy moves the
GWA to MIRROR, so the GWA has moved between consecutive MSA spectroscopy
activities even if they use the same science grating. Thus any MSA spectroscopy activity
should be followed by the lamp exposure if the confirmation image has been taken as part
of that activity.
5.1.2 Operational constraints:
Operational constraints on the lamps are described in Section 4.7.4 of the NIRSpec OCD
and as limitation ISIM-NRSI-L02 in the ISIM Constraints and Limitations Document
(JWST-REF-007587). The LINE lamps cannot remain on for more than 100 sec
(corresponding to NGROUPS = 10 in NRSRAPID), and once turned off each lamp must
remain off for 1000 sec to ensure temperature stability of the Fabry-Perot filters. Some
users may want to optimize their science exposures with this constraint in mind; APT
should help them do this. This constraint will also need to be implemented as a check in
Commanding.
Table 1 Auto wavecal lamp exposures for measuring the NIRSpec wavelength solution
Grating form Template
Lamp
NGROUPSa
G140M
LINE1 then REF
6 for LINE1, 6 for REF (TBR)
G140H
LINE1 then REF
6 for LINE1, 6 for REF (TBR)
G235M
LINE2
6 (TBR)
G235H
LINE2
6 (TBR)
G395M
LINE3
6 (TBR)
G395H
LINE3
6 (TBR)
PRISM
LINE4
6 (TBR)
a
NGROUPS may also need to be a function of aperture (TBD)
5.2
Auto flats
5.2.1 Operational details:
Auto flat lamp exposures depend on the band of the grating used in the corresponding
science exposure. The exposure parameters are valid for MSA spectroscopy only.
Because the wavelength-to-pixel mapping is always the same for the FS and IFU,
contemporaneous flats are not needed in those modes. All exposures use NRSRAPID,
NINT = 1, and full-frame readout. Auto flat exposures should occur when:
- the GWA position has changed since the previous MSA spectroscopy activity, or
- the MSA configuration has changed since the previous MSA spectroscopy activity.
The science use cases (see section 6) require flat-field data for each unique combination
of grating and MSA configuration. Implementing these flats for each change of grating
and MSA configuration in a visit could result in multiple auto flat exposures for the same
unique grating/MSA combination if a particular combination occurs at two nonCheck with the JWST SOCCER Database at: http://soccer.stsci.edu/DmsProdAgile/PLMServlet
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consecutive times within a visit. This repetition is acceptable scientifically (and could be
beneficial) and would be the recommended behavior if it is easier to implement in
commanding.
This implementation adds one parameter to the science templates for MSA spectroscopy:
AUTOFLATS = [YES, NO]. The default behavior will be decided as a policy matter but
we recommend defaulting to YES for the early portion of the mission, to build a database
of calibration information and ensure archive quality.
To minimize persistence effects, these auto flats should happen after the corresponding
science exposure and before any auto wavecals that are requested. Thus when both
AUTOFLATS and AUTOCALS are set to YES, the sequence would be science exposure,
auto flat, auto wavecal.
5.2.2 Operational constraints:
There are no specific operational constraints on the use of the FLAT lamps as there are
for the LINE lamps.
Table 2 Auto flat lamp expsures for measuring the NIRSpec MSA flat field response
Grating from Template
G140M
G140H
G235M
G235H
G395M
G395H
PRISM
Lamp
FLAT4
FLAT4
FLAT2
FLAT2
FLAT3
FLAT3
FLAT5
NGROUPS
20 (TBR)
20 (TBR)
20 (TBR)
20 (TBR)
20 (TBR)
20 (TBR)
20 (TBR)
6.0 Autocal Use Cases
6.1
Science + Autocal Use Cases:
6.1.1 Programs that require Autowavecals with Science
Science Case 1: Precise Kinematics of Stars in the Center of a Globular Cluster –
observe many point sources in a crowded region to obtain stellar radial velocities as
precisely as possible with NIRSpec (nominally ~10 km/sec) in the central region of a
globular cluster in order to evaluate the kinematics and measure the mass of any
intermediate mass central black hole. Multiple pointings with a single grating may be
needed, and multiple target acquisitions and/or multiple science target sets may be
observed within a single visit.
Autocal requirements:
Wavecals: With each grating movement wavelength calibration observations should be
embedded following or preceding science exposures in order to provide adequate
verification through each MSA slitlet of the wavelength zero-point and grating
positioning. Hence the capability for wavelength calibration exposures within a science
visit and presumably auto wavecal observations are required. In this case, both
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wavelength calibration zero-point and dispersion calibration are required by the science,
so that both FP and Er lamp exposures are necessary (AUTOWAVECAL=YES).
Flats: not specifically required (AUTOFLAT=NO).
Science Case 2: IFU Observations of AGN – obtain IFU observations of the central
regions of AGN with several gratings in order to carefully discern kinematics of the
nuclei. Precise characterization of the external wavelength calibration requires
observation of radial velocity (RV) standard targets, as well.
Autocal requirements:
Wavecals: Wavelength calibration observations must be embedded following science
exposures for both science targets and RV standard targets in order to place observations
on common precise system. Hence the capability for wavelength calibration exposures
within a science visit and presumably auto wavecal observations are required. Only the
FP lamp is required for this case as the science requires a wavelength dispersion relation
calibration (AUTOWAVECAL=YES).
Flats: not specifically required (AUTOFLAT=NO).
Science Case 3 (extension of scenario 204 in Soderblom et al, 2011): MSA Observations
to Obtain Kinematics of Galaxies in Clusters of Galaxies – obtain medium-resolution
spectra of galaxies to obtain accurate kinematics within representative clusters of
galaxies.
Autocal requirements:
Wavecals: In order to obtain precise wavelength calibration, wavelength calibration
observations must be embedded following science exposures. As persistence from
wavecal exposures could substantially compromise the science obtained for any
extremely faint sources, care may need to be exercised in the placement of the autocal
exposures. Hence the capability for wavelength calibration exposures within a science
visit and presumably auto wavecal observations are required. The science in this case
requires only wavelength zero-point calibration (AUTOWAVECAL=YES).
Flats: not specifically required (AUTOFLAT=NO).
6.1.2 Programs that do not require Auto wavecals or Auto flats with Science:
Science Case 4 (based on scenario 201 in Soderblom et al, 2011): Evolution of Ices in
Star-forming Environments – using multiple MSA configurations and two different
spectral elements observe several bright target sets in a heavily embedded star-forming
region in order to get accurate continuum measurements of point sources to measure
presence of ices in the cloud material.
Autocal requirements:
Wavecals: No precise wavelength calibration or auto-wavelength calibration
requirements as the strengths of well-defined continuum features will be measured. The
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targets for this project are quite bright, and science exposure times will be 60-100
seconds or less. The overheads associated with accompanying wave calibration
exposures are definitely not wanted for this case (AUTOWAVECAL=NO).
Flats: not specifically required (AUTOFLAT=NO).
6.1.3
Programs that do not require Auto wavecals, but do require Auto flats with
Science:
Science Case 5 (based on scenario 204 in Soderblom et al, 2011): First-light Galaxies in
the Hubble UDF – obtain medium-resolution spectra of the few extremely faint galaxies
in the UDF that have large photometric redshifts in order to confirm the nature of firstlight sources in the distant universe. As these galaxies are expected to be mainly
continuum sources and not necessarily Lyman-alpha sources, the goal here is to confirm
the photometric redshifts by detection of the Lyman break . Exposure times of order
several hundreds of hours will be required.
Autocal requirements:
Wavecals: No precise wavelength calibration or auto-wavelength calibration
requirements for the Lyman break detection and general confirmation of large
photometric redshift. Additionally, without careful planning persistence from wavecal
exposures would compromise the science obtained for these extremely faint sources
(AUTOWAVECAL=NO).
Flats: Proper throughput for specific shutters is required. Auto flat observation also
provides verification of the calibration model, particularly in the early mission. Flat
exposures should follow science as persistence from flat lamp exposure could
compromise science exposures on faint targets (AUTOFLAT=YES).
Science Case 6: Very High Flux (High S/N) Targets: Observe old red giant stars in the
galactic bulge to derive extremely accurate metallicities.
Autocal requirements:
Wavecals: No precise wavelength calibration or auto-wavelength calibration
requirements (AUTOWAVECAL=NO).
Flats: It is essential to obtain highly accurate flat fielding to properly assess weak
photospheric features, hence the capability for flat field calibration exposures or
preceding science exposures within a science visit is required. Pending results from FM
testing, persistence from very bright science sources could impact flat calibration
(AUTOFLAT=YES).
6.2
STScI Calibration + Autocal Use Cases:
6.2.1 Auto wavecal Use Cases
Calibration Case 1: RV Standard star observation through FS, IFU or MSA. Autowavecal observed with every grating reconfiguration (AUTOWAVECAL=YES) .
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Calibration Case 2: MSA Observations of emission lines in an extended region of a
planetary nebula. Auto-wavecal observed with every MSA reconfiguration
(AUTOWAVECAL= YES).
6.2.2 Auto flat Use Cases:
Calibration Case 3: Dithered MSA spectroscopic observations of an astrometric field to
construct an external pseudo L-flat; obtain a contemporaneous auto flat for direct
comparison and verification of measured throughputs within the MSA field
(AUTOFLAT=YES).
7.0 Autocal Considerations for Calibration
Implementation of automatic calibrations for all NIRSpec observing modes will provide
additional information on the wavelength calibration to verify the position sensor in the
grating wheel mechanism. In addition, automatic flat field exposures can be used to
verify the flux throughput character within NIRSpec, and autoflats acquired early in the
mission will be used to build-up the full instrument calibration model in the MSA
observing mode. As mentioned in section 5.1, auto wavecals in NIRSpec MSA
observing mode will be acquired after each MSA reconfiguration if the user requests that
a confirmation image is taken for the target placement verification. This is because it is
necessary to verify the wavelength zero-point after each grating move. In the MSA
mode, the wavelength calibration of a given target will depend upon the wavelength zeropoint, verified by the auto wavecal exposures, but will also depend on the target centering
of a source within the MSA shutter. As such, proper calibration of the wavelength scale
for NIRSpec MOS targets can be verified in the pipeline using the auto wavecal
exposures as well as the confirmation image exposures. Figure 1 presents the options
available to users for wavelength calibration of NIRSpec MSA mode observations, and
summarizes the methods for calibration in the pipeline.
Figure 1 Auto wavecal options for MSA Mode observations, and a summary of the resulting
calibration strategies.
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8.0 Autocal Operational Requirements Summary:
Several operational requirements and definitions are necessary in order to implement
autocals in a consistent manner. These include:
1. There should be a user option to take or not take auto wavecals
(AUTOWAVECAL=) and a separate user option to take or not take auto flats
(AUTOFLAT=) for any configuration. There will be "default" values for these
auto wavecal and auto flat options.
-­‐
Auto wavecals should be enabled (AUTOWAVECAL=YES) by default
(TBR) and the option to turn them off should be user-selectable.
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Auto flats should be enabled (AUTOFLAT=YES) by default and the option to
turn them off should be user-selectable.
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Users should NOT be allowed to specify the placement of autocals in an
observing sequence.
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Auto wavecal exposure durations will NOT be user-selectable, but may be
aperture-dependent (TBD).
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At this time it is TBD whether alternative auto flat exposure times from a
defined list (e.g., LONG, SHORT) may be necessary and user-selectable.
2. For AUTOFLAT=YES, any grating movement or a new MSA position at the
same grating position will require a new auto flat; that is, the precipitating event
or trigger for an auto flat is any change in either MSA shutter or grating wheel
position.
3. For AUTOWAVECAL=YES, any grating movement will require a new auto
wavecal; that is, the precipitating event or trigger for an auto wavecal is any
change in grating wheel position.
4. After a trigger occurs, autocals should be taken once per configuration.
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Use the rules from section 5.1.2 above for limitations on minimum separation
of FP wavecals (LINE1-4); there are no limits on Er wavecals (REF) or flat
exposures.
o If multiple Er cals or flats should be taken in a visit, it is TBD whether
there should be a minimum wait time before another calibration lamp
exposure is taken. At present, we believe that no wait is required.
5. Impact of persistence after autocals:
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The final recommended placement of auto wavecals and auto flats at the
beginning or end of an observing set of exposures with a specific
MSA+grating configuration is TBD pending results of FM testing. Rules for
placement of auto wavecals may be different from placement of auto flats.
Different approaches may be necessary for bright vs. faint targets (considering
the possible different relative impact of persistence). Conceivably, rules to
implement usage of auto wavecals or flats may be mode-dependent as well as
target brightness dependent.
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6.
9.0
If both wavecal lamp and flat lamp exposures are to be taken for the same
configuration, the flat lamp exposure should execute prior to any wavecal
exposure. Thus when both AUTOFLATS and AUTOCALS are set to YES,
the sequence would be science exposure, auto flat, auto wavecal.
Consistent options should be used across all templates.
References
2010, DRD-OPS-02, “NIRSpec OCD,” issue 6, section 4.7.4
2011, JWST-REF-007587, “ISIM Constraints and Limitations Document,” Rev. J, ISIMNRSI-L02
2005, JWST-RQMT-000835, “NIRSpec Functional Requirements Document,”NSFR-15
Ferruit, P., 2005, NIRS-CAL-TN-0001, “Review of the in-orbit wavelength calibration
for NIRSpec”
Soderblom, D.R., Beck, T., Gordon, K., Karakla, D., Keyes, T., Long, D., Muzerolle, J.,
Tumlinson, J., Valenti, J., 2011, JWST-STScI-002270,”Observing scenarios for
NIRSpec”
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