nirspec operations concept d r a f t
Post on 15-Jan-2016
47 Views
Preview:
DESCRIPTION
TRANSCRIPT
NIRSpec Operations ConceptD R A F T
Michael Regan, Jeff Valenti
(STScI)
Wolfram Freduling, Harald Kuntschner, Robert Fosbury
(ST-ECF)
May 2003 NIRSpec Operations Concept 2
Operations Concept: purpose
End-to-end view of instrument operationUsed to describe:
Instrument operational modesTarget acquisition strategy and spacecraft pointing
accuracy and stability needsInfluence of instrument stability and mechanism
repeatabilityCalibration implementationData-flow model and ground/flight s/w tasksMechanism and lamp usage estimates
May 2003 NIRSpec Operations Concept 3
Instrument schematic
Optical and mechanical subsystems
May 2003 NIRSpec Operations Concept 4
Observing strategies
NIRSpec lexiconAn Activity is a set of basic instrument operations that results in the
completion of a clearly identifiable task such as: a target acquisition, a wavelength calibration, science observation etc.
A spacecraft Visit is the contiguous period during which the pointing is controlled by a single guide star (set)
A visit will generally contain a number of different activities
NIRSpec field: all instantaneous FOV that can be accessed with a single guide star (set)
Target set: a group of sources that can be observed simultaneouslyThere can be multiple target sets within a NIRSpec fieldTarget sets for acquisition and science can be different
Association: a set of data that can be processed as a unit by the pipelineEach move of a NIRSpec spectral element defines a new
associationEach scientific target set implies a new association
May 2003 NIRSpec Operations Concept 5
Obs. strat.MSA configuration: a pattern of open/closed MSA facets designed to
match a given target set or calibration requirementAn association may include multiple MSA configurations
An Aperture pattern is a group of open facets comprising a single spectroscopic slit/aperture
Sub-aperture dithering may be used within an association to move targets within an aperture for each MSA configuration
Large-angle dithering with MSA reconfiguration may be used to span detector gaps and ameliorate the effects of bad pixels
Between two consecutive detector resets, non-destructive reads yield an Image sequence
At each dither location, multiple image sequences may be recorded
A single image sequence may not span two sub-aperture dither locations
A NIRSpec observing program will contain hierarchical arrangements of these elements
May 2003 NIRSpec Operations Concept 6
Obs. strat.Types of science observations
Slitless spectroscopyno TA required
Spectroscopy with fixed-slitsMulti-object spectroscopy with the MSAImaging spectroscopy with the integral-field unit (IFU)NIRSpec imaging (mirror) will be used primarily for
target acquisition and calibrationAn important requirement is the retention and
archiving of images taken through an MSA aperture pattern — used to analyse the positions of
objects within the apertures
May 2003 NIRSpec Operations Concept 7
Obs. strat.MSA apertures
The MSA will be n x 512 facets in the spectral direction by n x 256 facets in the spatial direction. Currently, n is expected to be either 2 or 4
Aperture width in the spectral direction is set by the spectral resolution requirement and will typically be 1.5/2 times as large in Band III (long wavelength) as in Band I
Aperture length in the spatial direction is set by the requirements for sky subtraction (small sources) or by the angular size of larger sources. It is restricted by the requirement for a high multiplex factor to avoid overlapping spectra
May 2003 NIRSpec Operations Concept 8
Obs. strat.
MSA configurationcan be specified as an n2 x 512 x 256 bit array that can
be compressed using either a generic or a tailored compression/encoding scheme
A standard (limited) set of aperture patterns for spectroscopy will have primary calibration data. Other aperture patterns will have secondary calibration data provided on a best effort basis
May 2003 NIRSpec Operations Concept 9
Obs. strat.
DitheringDithers are needed to obtain maximum
spectrophotmetric accuracy, maximum sensitivity, and to deal with focal plane defects
Why?Bad pixelsUncertainty in the dark currentFill in detector gapsSlit lossesFlat field uncertaintiesSpectral sampling
May 2003 NIRSpec Operations Concept 10
Obs. strat.Sub-aperture dithering
The (point-)source throughput for a given aperture is determined by wavelength — relative size of PSF — and position within the aperture
Due to quantization of aperture locations, sources will not generally be optimally placed wrt either the aperture boundary or the inter-facet bars
Sub-aperture dithering is a method of improving flux calibration by providing empirical constraints on source extent and position. Images of the field with NIRSpec or NIRCam can further improve accuracy
Dither patterns are chosen to optimise S/N subject to a constraint on the required flux calibration accuracy
May 2003 NIRSpec Operations Concept 11
Obs. strat.
Large-angle ditheringDithering with MSA reconfiguration for a given target
set will be carried out to ameliorate the effects of bad detector pixels and gaps between detector sub-arrays
The required telescope slews will require neither a new guide star acquisition nor a new target acquisition
May 2003 NIRSpec Operations Concept 12
Obs. strat.
Overhead cost of ditheringFor long observations there will effectively no cost
associated with ditheringIndividual exposure times are limited by cosmic
rays and dark current
For short observations there will be a tradeoffMSA reconfigurations reduce systematic errors but
result in lower signal-to-noiseSub-aperture dithers have a lower cost
May 2003 NIRSpec Operations Concept 13
Obs. strat.NIRSpec Activities
An activity encodes a combination of different low-level instrument operations in order to enable identifiable tasks such as: target acquisition, contemporaneous calibrations, science observations…
Envisaged activities include:
Measure mirror positionPurpose: map positions on FPA to positions in AFP
Method: use calibration lamp to image ‘L-shaped’ MSA apertures for x and y centroid determination
Goal: to avoid having to do this by maintaining stable mirror orientation to avoid global shifts of more than 0.2 detector pixels (2) along the dispersion direction and more than 0.2 detector pixels (2) along the spatial direction after a grating wheel move
May 2003 NIRSpec Operations Concept 14
Obs. strat.Select imaging mode
Purpose: To enable observer choice between unprotected and protected imaging modes depending on the absence or presence of bright objects (capable of increasing detector dark current) in the NIRSpec FOV
Method: Requirements derived from observer’s use of Observation Design Tool (ODT — this is not an instrument safety issue). MSA shutters closed in zones around bright sources while light path blocked by filter wheel
Target acquisitionPurpose: To position the reference target set accurately in the AFPMethod: Requires multiple reference sources to achieve desired accuracy. Alternatively,
dithered observations of fewer sources can be used. Requires 3 reads of either a full-frame image or a set of up to 16 sub-images centred on nominated acquisition targets. Needs CR removal, flat fielding, multiple image centroiding, application of a failure criterion (and retention of diagnostic information), application of mirror orientation correction, application of telescope offset
Direct imagePurpose: To confirm the presence of science targets in their correct positions in the AFP
after a target acquisition sequence. The accuracy of flux and wavelength calibration depends on how well the target positions are known.
Method: Use either a protected (by the filter wheel) on an unprotected (no filter wheel change) MSA reconfiguration to prepare for a direct image through the science apertures. At least 3 detector reads are needed to allow CR rejection
May 2003 NIRSpec Operations Concept 15
Obs. strat.Disperser selection
Purpose: To configure NIRSpec for spectroscopic observations of science targetsMethod: Command the appropriate dispersing element into the optical path. The correct
filter will already be in place after the confirmatory direct image is obtained
Wavelength calibrationPurpose: To determine the zero point of the wavelength scale for each disperser element
to an accuracy of better than 0.2 detector pixels (2). (The wavelength dispersion relation, i.e., the higher terms, is obtained as part of the annual calibration program)
Method: Image a calibration line source through a set of science apertures (and fixed slits) and a selected disperser element
Goal: To avoid having to do this if the dispersing element mechanism repositions to sufficient accuracy
Science observationsPurpose: To obtain a science observation of a target set with a given MSA configuration
and/or fixed slit and a given dispersing element. Note that a given target set can (and generally will) be observed with different dispersing elements and with MSA reconfigurations (large angle dithers)
Method: Carried out after all preparatory sequences. Can include sub-aperture dithers (<0.5 arcsec). MSA reconfigurations for large angle dithers will generally be carried out in an unprotected mode since a dispersing element is in place. However, a protected MSA reconfiguration, involving a filter wheel move, will also be available
May 2003 NIRSpec Operations Concept 16
Imaging and spectroscopic modesMode Wavelength Filter wheel AFP Grating wheel
Imaging 0.6 - 5µm Transparent TA configuration Mirror
Imaging >1.0µm Long pass I TA configuration Mirror
Imaging >1.7µm Long pass II TA configuration Mirror
Imaging >2.9µm Long pass III TA configuration Mirror
Spectroscopy 0.6 – 5.0 µm Transparent 200 mas MOS R=100
Spectroscopy 0.6 – 5.0 µm Transparent 200 mas single slit R=100
Spectroscopy 1.0 – 1.8 µm Long pass I 200 mas MOS R=1000
Spectroscopy 1.7 – 3.0 µm Long pass II 200 mas MOS R=1000
Spectroscopy 1.7 – 3.0 µm Long pass II 300 mas MOS R=1000
Spectroscopy 2.9 – 5.0 µm Long pass III 300 mas MOS R=1000
Spectroscopy 1.0 – 1.8 µm Long pass I 200 mas single slit R=1000
Spectroscopy 1.7 – 3.0 µm Long pass II 200 mas single slit R=1000
Spectroscopy 1.7 – 3.0 µm Long pass II 300 mas single slit R=1000
Spectroscopy 2.9 – 5.0 µm Long pass III 300 mas single slit R=1000
Spectroscopy 1.7 – 3.0 µm Long pass II 200 mas single slit R=3000
Spectroscopy 1.7 – 3.0 µm Long pass II 300 mas single slit R=3000
Spectroscopy 2.9 – 5.0 µm Long pass III 300 mas single slit R=3000
Spectroscopy 1.7 – 3.0 µm Long pass II IFU R=3000
Spectroscopy 2.9 – 5.0 µm Long pass III IFU R=3000
May 2003 NIRSpec Operations Concept 17
Detector Operations
NIRSpec will be highly detector noise limited in R > 1000 modes
Up-the-rump/MULTIACCUM sampling has been shown to be better than Fowler for detector noise limited observations
In addition, up-the-ramp sampling is more robust against cosmic rays
May 2003 NIRSpec Operations Concept 18
T2 T2 T2 T2T2
Samples
Groups
Reset
TIME
Sig
nal
Lev
elBaseline Readout Mode
May 2003 NIRSpec Operations Concept 19
T2 T2 T2 T2T2
Samples
Groups
Reset
TIME
Sig
nal
Lev
elAlternative Readout Mode
(depends on noise characteristics of flight electronics & detector)
May 2003 NIRSpec Operations Concept 20
Readout summary
Only one detector mode, MULTIACCUM, is needed for the full operation of NIRSpec
Detector readout mode
Parameters Baseline values Rationale
MULTIACCUM Frames per groupNumbers of groupsGroup spacing
4 or 1exposure_time/5050 seconds
Permits CR rejection on the ground. Other common exposure types, e.g. Fowler Sampling and Correlated Double Sampling, can be treated as special cases of MULTIACCUM
Sub-array x-positionSub-array y-positionSize of sub-array
Optimal for very bright sources. Full-frame readout is implemented by setting the sub-array size to the full SCA
May 2003 NIRSpec Operations Concept 21
Other parameters
Sub-array readoutMinimum 12 second exposure time is too long for many
sources
Sub-array readout will be needed
Sub-array must not be limited to a square (e.g., rectangle needed for bright, fixed-slit targets)
Only one sub-array at a time
Readout time = 12 x (number of pixels in subarray/8 million) seconds
May 2003 NIRSpec Operations Concept 22
Electronic Gain
Goal is to have only one gain setting for NIRSpecMaximum gain is set by Nyquist sampling single
sample read noise (~9e-) or ~4e-/ADU
Would like to be able to use entire full well ~ 90K – 200K e-
16 bit A/D values lead to 64K dynamic range
Saturated values can be reconstructed from early reads in up-the-ramp
A single gain of 1.5e- to 2.5e- will work
May 2003 NIRSpec Operations Concept 23
Integration Times
Type Exp time (s) No. groups Time between groups (s) No. samples
Acq Image 24 3 0 1
Cal Lamp 60 2 60 1
Science 500 11 38,0 1,4
Science 1000 21 38,0 1,4
Science 2000 41 38,0 1,4
Science 4000 81 38,0 1,4
Science 8000 161 38,0 1,4
May 2003 NIRSpec Operations Concept 24
Target acquisition — outlineNeed for robust and completely autonomous TA
Assumptions: independent of NIRCamuses pre-defined slit mask (i.e. not computed on-board!)absolute pointing good enough to place objects initially close to slitsTA determines x, y and roll angle offsets
Requirements for input from observers:Spacecraft roll requirementTarget set descriptions
Analysis of TA image:is a challenge because of big pixelsimpossible to achieve required accuracy using a single reference staron-board analysis of images must be able to:
identify reference starscompute offsetsflat fielding will be necessary
Error budget:Inherent in target set descriptionIntroduced by spacecraft/instrument
May 2003 NIRSpec Operations Concept 25
TA input from observer
Desired spacecraft roll angle ()Astrometry and photometry for a reference target set
The reference target set may contain science targets
A MSA configuration designed for this target setOptional MSA configuration to protect detector from
bright sources during TAIndication of whether the reference targets should be
used to determine ( x, y, or only ( x, y)
Exposure time for TA images
May 2003 NIRSpec Operations Concept 26
TA goals
The MSA configuration requires targets within a zone of half a slit-width (spectral direction) centred on the aperture pattern
This is called the Nearest Facet Trigger Zone (NFTZ)
Errors in the TA or the target set astrometry can place targets outside their intended NFTZ
A successful TA places more than 75% of targets within the NFTZ in more than 95% of cases
With a typical Band I slit-width of 200mas, this leads to a requirement of a TA error of 25 mas (2) or 12 mas (1)
May 2003 NIRSpec Operations Concept 27
Throughput impact of TA errors
Typical TP variation is 10% for 12mas offset
Average TP loss for a target set for a 12mas offset is ~ 5% at 2µm
May 2003 NIRSpec Operations Concept 28
TA procedure (MSA)Telescope is commanded to go to NIRSpec fieldTelescope slewsDuring the slew, NIRSPEC is configured for target acquisition and the position of the MSA relative to the detector is calibrated. This consists of the following steps:
•Rotate the grating wheel into mirror position. NIRSPEC is now in imaging mode•Rotate filter wheel to diffuser/dark position•Turn on appropriate continuum lamp•Take short exposure•Read a window on the detector which is centered on target acquisition aperture•Send image to computer•Turn off lamp•Computer analyses the images and computes position of MSA relative to detector
Configure MSA for imaging of reference target set: all MSA shutters are opened (optionally shutters around bright objects are closed)After the telescope arrives at the target field, a MULTIACCUM 3 exposure is taken. This is nominally a reset followed by three reads of the detectorsImages are sent to computerFor each image, computer
•Divides by flat (TBC)•Takes the minimum of the read 1 – read 0, and read 2 – read 1 differences (CR reject)•Determines CoG of targets•Computes position of objects from CoGs•Computes x pixel, y pixel, angle (TBC)•If specified by observer, = 0•Using the previously computed position of the MSA relative to the detector, the xy offset is computed•Commands offset x pixel, y pixel, (TBC)
May 2003 NIRSpec Operations Concept 29
TA procedure - continued
Telescope slews to final positionMSA is configured to slit mask for science target setScience observations start
The first image in the science sequence will in most cases be an undispersed image to be used to aid the extraction of spectra
The TA procedure for the fixed slits will be identical — with the open MSA being used to determine offsets from a reference target set
May 2003 NIRSpec Operations Concept 30
TA error budget
The final positioning of a target set within the set of aperture patterns (slit mask) will depend on the accuracy of the supplied target coordinates relative to the reference target set (random?) and the accuracy of the TA procedure (bulk offset)
The TA accuracy depends critically on the number of reference targets and the requirements cannot be met with a single target
May 2003 NIRSpec Operations Concept 31
Centering through the MSA
Microshutter grid/detetector pixellation will lead to biases in the centroid of an individual point source ~14mas
More sophisticated algorithms can reduce this
Only by dithering one source or using multiple reference objects can this be averaged out
With 9 reference targets, a final error of 5 mas can be achieved
May 2003 NIRSpec Operations Concept 32
TA error budgetThis represents a
bulk shift/rotation between the MSA aperture patterns (slits) relative to the mean target set position
May 2003 NIRSpec Operations Concept 33
Errors in placement of individual science targets on associated slit
Arise from:
Errors in knowledge of target positions
Difference between actual roll angle and that assumed for the mask designTotal roll angle error budget assumed to be 10 arcsec
May 2003 NIRSpec Operations Concept 34
Source of target coordinates
Generally from NIRCam images
Roll angle is most critical factorThis means that a JWST roll angle error enters twice
into a NIRSpec observation — once from Cam and once in Spec => absolute roll requirement for JWST is 10/√2 = 7 arcsec
This absolute roll requirement is alleviated if the TA procedure can compute and apply a
Non-JWST target set sources are possible but difficult
May 2003 NIRSpec Operations Concept 35
Image Stability
Around 1/3 of the science will be one day per grating selectionNeed to be stable on this timescaleOtherwise, will have to reacquire and recalibrateSpacecraft roll about FGS star will need to be stable to
within ~3 arcsec per daySmaller due to larger radius to FGS star
It is vital that a series of small offsets (for sub-aperture and large-angle dithering) do not produce a cumulative error (JWST Lev 2)
May 2003 NIRSpec Operations Concept 36
Calibration goals
To allow the determination, for each observed target, the intensity of radiation as a function of wavelength and position along the spatial direction of the slitProvide reference files for all standard approved science modes
Provide reference files to enable all science operations such as TA
Monitor instrument status and performance
Minimise on-orbit calibration time investment
Maximise utility of general science calibrations
Minimise science programme-specific calibrations
May 2003 NIRSpec Operations Concept 37
Calibration — science requirements
Derived from reference science programmes (Kuntschner et al. 2003)Wavelength: The combination of systematic and relative errors in the wavelength calibration
will be smaller than 1/10 (rms) of the characteristic resolution element (FWHM) for a given grating/prism, over the full wavelength range and FOV
Spectrophotometric: Assuming no Poisson noise in the signal, multiple observations of the same target with different MSA, fixed slit or IFU configurations will provide a repeatability for the overall throughput (with respect to a given standard source) of better than 5% (rms) for the full FOV. The throughput uncertainty as a function of wavelength will be below 5% (rms)
Spatial: The spatial coordinate along the slit will be known to better than 20mas (rms) with respect to the coordinate frame defined by the target acquisition reference objects. This accuracy will be met at all wavelengths, over the full FOV.
Spatial PSF: The spatial PSF shape (relative intensity of a point source along the spatial direction of the slit) will be known to better than 3% (rms) at all wavelength and the full FOV
Spectral PSF: The line spread function (relative intensity distribution of a delta function along the dispersion direction) will be known to better than 3% (rms) along the dispersion direction at all wavelengths and the full FOV
May 2003 NIRSpec Operations Concept 38
Calibration types
NIRSpec will be able to produce all required performance and calibration data using a combination of the following on-board calibration types: Pointed calibrations: Dedicated observations of astronomical objects. For
some observations an accurate TA may be needed
Sky calibrations: Observations of typical sky (i.e. background) regions. Normal telescope pointing will be sufficient
Lamp calibrations: Internal continuum and line lamp observations
Dark calibrations: Observations with the light path blocked. Auto and Opportunistic calibrations: The calibrations make use of the science data itself (auto) or are extracted from another set of suitable science observations (opportunistic). These calibrations do not require specific observations
Pointed and sky calibrations require NIRSpec to be the primary instrument, while dark and lamp calibrations are suitable to be carried out in parallel mode
May 2003 NIRSpec Operations Concept 39
On-board lamps
NIRSpec will be equipped with internal line and continuum lamps
The brightness of the signal produced by the calibration system must be stable (limits TBC) and predictable
Pre-planned power settings and exposure times must result in data quality that satisfies the needs of the exposure
It will be possible to use the calibration system with any allowed spectroscopic mode and produce a signal which satisfies the calibration requirements
Detector over-illumination (producing persistence) will be avoided
The calibration subsystem should be capable of use in parallel with other JWST science instrument operations
May 2003 NIRSpec Operations Concept 40
The exposure time required to produce the signal needed for calibration purposes will ≤ 60s for the wavelength calibration system and ≤ 10 x 60s for the continuum calibration
The line lamps will produce > 10 lines uniformly covering the full wavelength range of each dispersing element with a S/N > 30 in each of the lines
The continuum lamps will provide a S/N > 500 over the full wavelength range and FOV of each dispersing element
The stability of the lamps will be such that after the nominal lifetime of JWST the required signal is still achieved within the nominal exposure times listed above
May 2003 NIRSpec Operations Concept 41
Science program specific calibrations
Will consist of the following types: 1. “Through slit” images of the target(s) after a
successful target acquisition
2. Wavelength zero-point calibration
It is assumed that the spacecraft and instrument stability are such that these do not need to be repeated during observations of a particular target set with a given disperser within a particular visit
May 2003 NIRSpec Operations Concept 42
Monitoring calibrations
Monitoring calibrations scheduled at regular time intervals for use by all regular science programs
NIRSpec offers a limited number of observing modesBut monitoring calibrations needed for large set of MSA Implies need for highly stable and well behaved spectrograph:
geometric distortions, sensitivity and wavelength solution should change only smoothly across the FOV.
Small scale flat-field (FF) calibration strategy relies on an extensive pre-flight calibration of the detectors. A determination of the full pixel-to-pixel FF as a function of wavelength is impossible in orbit (e.g., lack of narrow band filters, Kuntschner et al. 2003).
For efficiency, must separate monitoring calibrations into those that can be done in parallel and those where NIRSpec is required to be the primary instrumentThe loss in science time without parallel capabilities is estimated to be at least 5%
of total NIRSpec time
May 2003 NIRSpec Operations Concept 43
Measurement Type Frequency per year
Dark current Dark 2
RN determination Dark 2
RN verification Dark 52
Hot pixels Dark 120
Facet throughput continuum lamp 12
Conventional slit throughput continuum lamp 2
Small scale flat field continuum lamp 12
Geometric distortions continuum lamp 1
Slit positions continuum lamp 1
Spectral trace continuum lamp 1
Dispersion solution continuum lamp 1
Slit contrast continuum lamp 1
Detector gaps continuum lamp 4
Gain continuum lamp 2
Parallel-capable calibrations
May 2003 NIRSpec Operations Concept 44
Measurement Type Frequency per year
Linearity Pointed 1
Image persistence Pointed 12
PSF Pointed 1
Image anomalies Pointed 1
Photometric response Pointed 1
Geometric distortions Pointed 1
Focal plane position Pointed 2
Line spread function Pointed 1
Line lamps Pointed 1
Large scale Flat Field Sky 12
NIRSpec primary calibrations
May 2003 NIRSpec Operations Concept 45
Data-flow model
Need for a coherent environment to handle complex observation design process and the resulting properly-described data structures
Outlines needs for ground and flight s/w
May 2003 NIRSpec Operations Concept 46
Usage estimates
1. Thermal worst-case day
2. Mission lifetime
May 2003 NIRSpec Operations Concept 47
Thermal worst-case day
AssumptionsSpectroscopic survey of bright sources
Lowest ratio of integration time to mechanism moves
Assume that only the load on 24 hr period is important
30 minutes of integration time per source
Subsequent fields are nearby so slew time is small
Three MSA configurations per visit
May 2003 NIRSpec Operations Concept 48
Visit setup after first visit
Task Time FW move GW move
MSA config Lamp
Slew (small) 1 min 0 0 0 0
GS acq 2 min 0 0 0 0
Take acq. image
3 min 1 1 1 1
Calc offset 10 sec 0 0 0 0
Move 1 min 0 0 0 0
May 2003 NIRSpec Operations Concept 49
Science tasks
Task Time FW GW MSA Lamp
Setup 3.5 min 3 1 1 1
Observe 10 min 0 0 0 0
Dither 1 min 0 0 0 0
Config MSA 30 sec 0 0 1 0
Observe 10 min 0 0 0 0
Dither 1 min 0 0 0 0
Config MSA 30 sec 0 0 1 0
Observe 10 min 0 0 0 0
May 2003 NIRSpec Operations Concept 50
Worst-case daily load
How long for one visit?45.6 min or 0.032 day
33 visits per day
What does this mean?Filter wheel moves – 63 times/day (every 1400 seconds)
Grating wheel moves – 63 times/day (every 1400 seconds)
MSA configures – 126 times/day (every 720 seconds)
Line Lamp – 63 times/day (every 1400 seconds)
May 2003 NIRSpec Operations Concept 51
Mission lifetime usages
Assumptions5 year lifetime
NIRSpec is used 50% of the time
90% of the time we are doing science
All observations are multi-object MSA spectroscopy
May 2003 NIRSpec Operations Concept 52
Type of projects
Type Fraction of NIRSpec
time
Average time for one visit
(Ks)
Number of visits per year
Short 0.10 3 370
Medium 0.60 20 330
Long 0.30 100 33
May 2003 NIRSpec Operations Concept 53
Usage for each visit
Type Filter Wheel Grating Wheel MSA Lamp
Short 9 4 8 4
Medium 7 3 6 3
Long 5 2 4 2
May 2003 NIRSpec Operations Concept 54
Total usage (5 years)
Type Filter Wheel Grating Wheel MSA Lamp
Short 5x370x9 5x370x4 5x370x8 5x370x4
Medium 5x330x7 5x330x3 5x330x6 5x330x3
Long 5x33x5 5x33x2 5x33x4 5x33x2
Total 29K 13K 25K 13K
May 2003 NIRSpec Operations Concept 55
Action Items
L2 requirement of 1% positional error for any moves > 0.5”, would lead to NIRSpec target acq after ditherLine-of-sight working groupRegan to write TPRR on L2 requirement
Memory requirements for arbitrary MSA masks are largeValenti/Regan to come up with compressed version
Is a flat field required for target acquisition?OPSCON Team
Detail upload information for target acquisition.OPSCON Team
Is there are requirement for “light friendliness”?GSFC
top related