nirspec operations concept d r a f t

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


Instrument schematic

Optical and mechanical subsystems


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


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


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


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


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


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


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


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


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


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


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


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


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 2.9 – 5.0 µm Long pass III 300 mas single slit R=1000



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


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


T2 T2 T2 T2T2

Samples

Groups

Reset

TIME

Sig

nal

Lev

elBaseline Readout Mode


T2 T2 T2 T2T2

Samples

Groups

Reset

TIME

Sig

nal

Lev

elAlternative Readout Mode

(depends on noise characteristics of flight electronics & detector)


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


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


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


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


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


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


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)


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


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)


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


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


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


TA error budgetThis represents a

bulk shift/rotation between the MSA aperture patterns (slits) relative to the mean target set position


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


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


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)


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


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


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


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


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


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


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


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


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


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


Usage estimates

1. Thermal worst-case day

2. Mission lifetime


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


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


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


Dither 1 min 0 0 0 0

Config MSA 30 sec 0 0 1 0



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)


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


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


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


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


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

nirspec operations concept d r a f t

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