TECHNICAL REPORT

TECHNICAL

REPORT

Title: Alternative Strategies for NIRSpec

Target Acquisition in FS and IFU Modes

Doc #:

Date:

Authors: T. Beck Phone: 410-

338-5038

Rev:

JWST-STScI-001751, SM-12

4 May 2009

-

Release Date: 8 September 2009

1.0

Abstract

At the present time, only one target acquisition (TA) procedure for all science modes is defined for NIRSpec. It has been assumed that this method of TA will encompass all observations in all modes, so a backup means to acquire NIRSpec science targets has never been defined. However, there are many situations where this standard TA will not be optimal from logistical or practicality standpoints. This is highlighted by the fact that standard NIRSpec TA requires high resolution images for reference target definition, which will complicate scheduling if every NIRSpec science program requires NIRCam pre-imaging prior to execution. There are also science use cases for NIRSpec – for example the bright object, ‘wide aperture’ science for planet transit spectroscopy – where it is not presently possible to do TA for the science observation. This report outlines the standard NIRSpec TA, identifies situations where this will not be optimal (or possible), suggests alternative solutions, and recommends priorities for implementation.

2.0

Introduction

The standard NIRSpec TA method uses reference stars observed through the field sampled by the MSA quadrants. Images of the reference stars are obtained through the

MSA, and the NIRSpec TA onboard script iterates on the measured coordinate positions until the final rms accuracy converges to <20mas. A very high spatial resolution IR preimage must be used to define the positions of the reference stars and science targets for this TA to succeed (likely a NIRCam image). This is the only method for TA that is presently defined for the MOS, fixed slit (FS) and integral field unit (IFU) science modes with NIRSpec.

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The specific goals of this study are to: o Define means to acquire a target in the IFU or FS modes of NIRSpec so that high spatial resolution near IR pre-imaging is not always required. o Propose methods to acquire a target in the IFU or FS modes of NIRSpec that do not rely on the MSA at any point in the process, in the event of MSA quadrant failures or failures in the standard TA. o Define methods to acquire targets in operational modes where the standard NIRSpec

TA through the MSA will be difficult or impossible – namely for moving targets and bright object/wide aperture science.

In the Fall 2008, a change was made to the NIRSpec MSA mounting plate. Figure 1 presents the region of the mounting plate in between MSA quadrants 3 and 4, and identifies the location of the IFU entrance aperture and the FSs. There is now a

1.”6x1.”6 square aperture in place of the 0.”1 fixed slit (a.k.a. – wide aperture; Figure 1).

This alteration was requested by the JWST science working group for very high signalto-noise spectroscopy of bright stars to detect and characterize planetary transit events.

In addition to this important science application, the new square aperture provides a means to measure and monitor the NIRSpec PSF in a small imaging field without the

MSA bars partially obscuring the view. As proposed below, this 1.”6x1.”6 aperture will also prove very useful for target acquisition without using the MSA quadrants for the IFU and FS science modes.

Figure 1: A zoomed view of the MSA quadrant mounting plate in the region of the fixed slits. The MSA quadrants, IFU entrance aperture, Fixed Slits, and new 1."6 square wide aperture are labeled.

3.0

Standard NIRSpec Target Acquisition

The standard NIRSpec TA is defined in detail in the NIRSpec OCD and in the requirements document “JWST NIR-Infrared Spectrograph (NIRSpec) Target

Acquisition Requirements” (JWST-RQMT-006993). The NIRSpec TA procedure uses 8-

20 reference stars with accurate astrometry (<5mas) observed throughout the MSA using



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SM-12 the imaging mode, calculates centroids of the individual stars on the detector, transforms their pixel coordinate positions into positions on the sky, and iterates on the telescope pointing and slew until the rms position of the reference stars is less than 20mas. To mitigate the effect of the MSA bars on reference source positions, two TA images will be obtained – one at the nominal pointing, and one offset by ½ MSA shutter pitch in x and y.

A detailed analysis of the sources of uncertainty in the delivered target acquisition RMS are included in the report “Error Budget for NIRSpec Target Acquisition” (Jakobsen

2005b; JWST-REF-05938). This standard TA procedure is outlined in more detail in

Figure 2 and the description below.

Figure 2: NIRSpec Sandard Target Acquisition flow chart. o Step 1: Slew to target position and roll angle. Configure the MSA for imaging mode

(grating mirror in). Take an internal calibration exposure with the CAA lamp to locate and centroid the position of the fixed slit used for reference. Determine and apply any mirror offset. o Step 2: Configure the MSA to all open (or close any shutters on bright targets identified by a MSA configuration file which might saturate the detector during the acquisition process). Take two external acquisition images of three reads each (to



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SM-12 correct out CRs), separated in pointing by half of a micro-shutter pitch (~120mas in x,

250mas in y). Correct the images for flat field variations. o Step 3: In the onboard scripts, identify and centroid the reference stars in each image.

The locations of the reference stars will be measured in 32x32 pixel sub-array boxes centered on the expected position of the star. Transform the position of the stars on the detector to their undistorted positions on the sky. Calculate the least squares offset, and calculate and check the residuals (which requires very precise knowledge of the combined effects of distortion in NIRSpec). Repeat the calculation and recheck the residuals until they are <20mas. o Step 4: Comparison of the measured reference star positions on the sky with the

“ideal” positions specified by the observer during the MSA planning stages will allow the onboard scripts to compute the final corrective spacecraft slew. Execute the slew. o Step 5: Take an image to verify the TA (the “reference image”).

All of the NIRSpec observing modes (MSA, FS and IFU spectroscopy) are currently required to use this standard TA. The locations of the NIRSpec TA reference stars and science target positions will be defined by GOs using the NIRSpec MSA Observation

Planning Tool in the APT (Valenti et al. 2006). At the present time, very high resolution infrared images with accurate astrometry (<5mas) are required to define the positions of the reference acquisition stars for all TA activities, so infrared pre-imaging must be acquired before execution of the spectroscopy. This effectively means that JWST

NIRCam or HST WFC3 images of the target field must exist prior to the definition of

NIRSpec spectroscopic science programs in the APT. Many programs will not have high resolution infrared images available, and in these cases NIRCam pre-imaging would always be needed as part of the NIRSpec program to execute the standard TA in the

NIRSpec FS, IFU and MOS modes.

As a side note, JWST will be operating in the era of high spatial resolution ground based multi-conjugate adaptive optics (MCAO) infrared imaging of wide-fields. It is conceivable that PIs will wish to use their previously acquired ground-based AO images for NIRSpec pre-imaging. The relative and absolute astrometric accuracy of ground based wide-field imaging from general observatories is often insufficient for standard

NIRSpec TA. Use of ground-based MCAO images for standard NIRSpec TA should not be allowed, unless the astrometric accuracy of the instrument in question has been well demonstrated. Other wide-field seeing-limited IR imaging from ground-based observatories may not have the astrometric accuracy or sensitivity necessary to derive very accurate science target centroid positions relative to reference stars. The use of ground based pre-imaging should be discouraged for standard NIRSpec TA.

3.1

NIRSpec Observing Modes Where the “Standard” TA Procedure MUST be

Used

Many observations with NIRSpec will require the standard TA procedure described in the preceding section, and will therefore need a high spatial resolution infrared pre-image of the science field. Situations where pre-imaging observations are required include: o All MSA Observations



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SM-12 o Any IFU or FS Observations of crowded fields where TA requires very high spatial resolution imaging and accurate astrometry for centroiding on the science target in high stellar density regions. The NIRSpec team shall provide users with detailed guidelines when IFU and FS TA will require pre-imaging due to difficulty of setup. o Any FS or IFU program where the PI requests pre-imaging and standard TA, with the understanding that the time used for the imaging comes out of the science program allocation.

3.2

Situations where “Standard” Target Acquisition through the MSA is not

Optimal

All MSA mode observations will use the standard TA procedure to attain the target centering accuracies necessary for MSA science. Additionally, IFU or FS observations in crowded fields may demand NIRCam pre-imaging to define the positions of the TA reference stars when accurate coordinates are not available. Of course, pre-imaging can always be acquired if the program PI requests it. However, pre-imaging (or very high spatial resolution HST or JWST images) should not be necessary in order to execute all science observations using the IFU or FSs – particularly on bright targets. Moreover, there are limitations that should be considered if NIRCam pre-imaging observations for

TA are required on all NIRSpec science programs: o Observation Definition: If all NIRSpec observations require high spatial resolution pre-images, then it will be difficult or impossible to define and construct executable

NIRSpec science programs at the Phase I deadline. Phase II programs must exist for proper definition of NIRSpec TA after pre-imaging has been acquired. Given the present definition of TA, pre-imaging and Phase II deadlines will exist for nearly all

NIRSpec science programs. o Scheduling: Pre-imaging observations must be scheduled at the beginning of an observing cycle or at the beginning of the time period when the science target is observable. GOs will need ~1 month of time (TBD) after pre-imaging to construct their Phase II programs for science observations (MOS, IFU or FS). If the majority of NIRSpec science requires pre-imaging, and if it takes a month of turn-around time for post-imaging Phase II deadlines, then it may be quite difficult to schedule any

NIRSpec spectroscopy programs within the first ~1 month of an observing cycle.

NIRSpec observations will be always be relegated to the latter parts of the JWST science cycles, which could impede scheduling programs late in the observing sessions (This is especially true given that NIRSpec MOS observations will be further constrained to a ‘fixed orient’ at the Phase II deadline). o Lack of necessity for IFU/FS Modes: For observations of FS or IFU targets, the primary reason for requiring NIRCam pre-imaging observations would be to define very accurate coordinates of reference stars for target acquisition through the MSA.

There is little need to use NIRCam pre-images to define the positions or fluxes of most FS and IFU science targets. Moreover, the <20mas setup accuracy required for the NIRSpec MOS spectroscopy target acquisition particularly does not apply to observations with the IFU, where target centering to an accuracy of only ~0.”1 is sufficient. Alternative TA strategies examined in later sections of this report are



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SM-12 preferable. TA using the new wide aperture or with a “peak-up” algorithm as presently used for HST STIS narrow slits might suffice and may even be more efficient for centering bright targets in the NIRSpec FSs. o Failed Closed Shutter Effects on TA: The NIRSpec MSA has many failed closed shutters that will affect accurate centroid measurements on reference stars in the standard TA method. Setup of MOS spectroscopic observations can get around this easily, if a failed closed shutter lies near or on a reference star the field can be translated by a small x or y offset to move the reference star and MOS targets away from the failed shutters. However, FS and IFU targets cannot be similarly translated, the science target has to be aligned exactly with the science aperture. Small orient changes may get around this. However, because of the failed closed shutters it could prove difficult to acquire accurate centroids on 8 to 20 reference stars for FS and IFU targets using the standard TA method.

Furthermore, there are situations where the use of the standard MSA target acquisition method for all NIRSpec observations will be impractical (or even impossible) to implement: o Wide Aperture Spectroscopy (Planet Transits): It will not be possible to center bright stars within the wide 1.”6 x 1.”6 square aperture using the standard NIRSpec target acquisition method described in section 3. This requires full-frame readout of the detector, and the bright stars will badly saturate in the aperture and may cause persistence effects in the resulting science exposures. The bright star will also strongly saturate in any NIRCam pre-imaging observation, making accurate (<20mas) centroid determination impossible and the identification of nearby faint (J<16) reference stars difficult.

o Moving Targets: Executing the standard target acquisition method for non-sidereal targets is likely to be complicated and difficult because the TA reference stars will be continuously moving with respect to the science source. o Rapid Turn-around, “Target of Opportunity” Observations: Requiring preimaging for reference target location for all NIRSpec science observations will preclude rapid turn-around observations of astronomical “targets of opportunity”, such as gamma ray bursts, supernovae and solar system events.

Many PIs will not wish to use their time allocations to acquire NIRCam images of their fields that are not required, and unnecessary pre-imaging may make NIRSpec science programs more difficult to schedule in JWST observing cycles. For some

NIRSpec observing modes, the standard NIRSpec TA will not provide a sensible means to center the science target in the requested IFU or FS aperture. The general problem of target saturation during TA described above for the wide aperture planet transit case is also applicable for general bright object FS science that will be observed with a sub-array to limit saturation effects in the spectra (this an issue for FS targets with J<~15.9; TBD).

As a result, relaxed constraints on the standard TA and methods for autonomous setup on bright IFU or FS science objects are desirable without requiring high resolution preimaging observations. Defining alternative methods for TA is also critical in the event of a failure of the MSA. As target acquisition is presently defined (section 3), if the MSA



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SM-12 quadrants fail on-orbit then NIRSpec will have no means to execute target acquisition for

IFU or FS mode science.

4.0

NIRSpec Target Acquisition Options with no Pre-Imaging

This section describes methods to autonomously setup NIRSpec IFU and/or FS observations without requiring high resolution infrared pre-imaging observations. Unless stated explicitly otherwise, we assume that the standard data acquisition philosophy and centroiding method put forth by Valenti, Regan and Balzano (2007) will be used.

Namely - all “acquisition images” are taken with three frames in the up-the-ramp

MULTIACCUM pattern so that two difference images can be constructed and cosmic rays (CRs) can be rejected. It is also assumed that TA data will be acquired using the standard readout patterns that are already defined for NIRSpec (NRSRAPID, NRS and

NRSSLOW).

4.1

NIRSpec Standard TA with Coarse Accuracy (Standard IFU/Wide Aperture

Acquisition)

In the absence of NIRCam pre-imaging, very high resolution HST images from WFC3

(optimal), ACS or WFPC2 could be used for NIRSpec acquisition planning. Of course,

ACS and WFPC2 images will provide accurate reference object coordinates at optical wavelengths only, so approximate translation relations from optical to IR colors could be used to gauge the IR flux of the reference stars used for TA (Chayer 2008). At the beginning of the mission, NIRSpec MSA TA will use many WFC3 images, but this may not be the case for FS and IFU Modes.

Ideally, we seek to define a method to carry out target acquisition for NIRSpec science that:

1) Does not require PI allocated time be spent to obtain a NIRCam pre-image.

2) Requires a minimal amount of changes to the existing “standard” TA process and existing FSW.

3) Results in a TA accuracy of better than ~100mas that would serve well for IFU and faint object science programs with the wide aperture.

Target acquisition for IFU observations does not require the delivered accuracy of <20mas as needed for MOS spectroscopy because the IFU field is 3 ” x 3 ” in extent and slight offsets in target centering will not affect the flux throughput of most science sources.

Moreover, wide aperture observations of faint sources also do not have a stringent centering requirement. A delivered set-up accuracy of 100mas rms is sufficient for TA for IFU and wide aperture faint object spectroscopy.

In the cases where both NIRCam and HST imaging of a target field do not exist, catalog coordinates could be used to define the positions of target acquisition reference stars, with resulting decreased accuracy in the final TA centroiding. The most accurate astrometric catalogs with the widest sky coverage that can be used for less but sufficiently accurate TA will be obtained from the SDSS or 2MASS catalogs. The typical rms astrometric accuracies are 6-10 times better for SDSS and 2MASS compared to GSC-II. A summary of the SDSS and 2MASS projects and astrometric accuracies is included in Table 4-1.



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Figure 3 presents a plot of the delivered RMS target acquisition accuracy as a function of the number of TA reference stars used and the input catalog coordinate accuracy. This figure was constructed using the error estimation method for standard TA put forth by

Jakobsen (2005b). As expected, for a given delivered TA rms, the necessary input catalog coordinate accuracy becomes more strict with fewer reference stars used.

Overplotted on the figure are lines which show the RMS accuracy of the SDSS, 2MASS and USNO astrometric catalogs. Sources in the USNO catalog have higher RMS accuracy and are too bright to be useful for NIRSpec TA. However, the IR coordinates for the 2MASS J-band point source catalog are accurate to rms ~70-80 mas for J<16, and

SDSS optical coordinates are accurate to rms ~45 mas for r < 20 (Table 4-1).

Figure 3 shows that SDSS or 2MASS catalog coordinates using 2 or more TA reference stars will deliver an RMS setup accuracy of less than 100mas. Ideally, NIRSpec users can identify their reference setup stars using these catalog coordinates, and the target acquisition will execute the steps of the standard acquisition procedure (section 3) and iterate on the centroid calculation until the residual is better than 100mas only (instead of

<20mas). Hence, TA can be accomplished with no pre-imaging and little adaptation to the existing TA procedure by using the standard method with decreased accuracy SDSS or 2MASS catalog coordinates on a minimum of ~3-5 reference stars. The use of

2MASS and SDSS coordinates to define reference star positions in place of HST or

NIRCam pre-images will provide coarse centering of a target, and as such will be applicable primarily for target acquisition with the IFU and for faint targets observed through the 1.”6 square aperture (the latter of which are not in danger of saturating the detector in the time it takes for full-frame readout). This course centering could thus be a powerful method for TA on faint moving targets observed through the wide aperture, such as KBOs where the orbits are not well determined.



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Figure 3: The required astrometric coordinate accuracy plotted versus the number of reference stars for achieving levels of delivered TA accuracy.

Table 4-1 Astrometric Catalog Parameters

Catalog Photometric

Bands

SDSS r, i, z

Faint

Limit r~22

Astrometric

Accuracy

45mas@r~20

Sky

Coverage

25%

Reference

Pier et al. 2003

2MASS J, H, Ks Ks~16 70-80mas@K<15 70% McCallon 2002

The 2MASS J-band coordinates (at J<16mag) could provide a useful overlap with

NIRSpec J-band target acquisition filters, and can allow for high S/N centroid calculation on reference sources. Jakobsen (2005b) calculates saturation magnitudes for the standard

TA using the narrow NIRSpec TA filter that are at the faint limit of the 2MASS catalog

(J~15.9 mag). We anticipate that the faintest sources with 2MASS J-band catalog coordinates will provide accurate reference star coordinates for TA. However, further updates on NIRSpec sensitivity estimates and detector well depth are necessary to determine the overlapping range of J-band magnitudes that are acceptable for coarse accuracy TA using this method.

In the future, deeper wide field infrared imaging surveys such as the WFCAM (UKIRT; northern hemisphere) survey and the VISTA survey telescope (ESO; southern



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SM-12 hemisphere) may provide catalogs of accurately calibrated IR coordinates to fainter limiting magnitudes than 2MASS. It may be possible to incorporate use of the WFCAM or VISTA survey catalogs for this coarse accuracy IFU and wide aperture target acquisition. Ground-based wide field IR imaging may also be used for this coarse accuracy TA, if the astrometric accuracy of the reference star coordinates is better than

~80 mas (Figure 3).

This coarse accuracy centering method can also be used to place a science target within

~100mas of one of the narrow FSs (0.”2 width). A FS peak-up (see section 5.3) could then be run to very accurately center the source within the slit. This may provide an important means to execute 0.”2 FS TA on bright objects and moving targets.

4.2

Centering a Target in the 1.”6x1.”6 Wide Aperture

As mentioned in section 2, the NIRSpec MSA mounting plate has been altered to include a 1.”6 x 1.”6 square aperture (the 0.”1 slit has been removed). This wide aperture will provide the primary method for executing target acquisition without using the MSA at any point in the process. This is very important for FS or IFU TA in the unlikely event of a failure of the MSA.

The RMS slew accuracy from a prior pointing to a new NIRSpec target is ~0.”6. Hence, the 1.”6 square aperture does not sample a large enough area of the sky to provide a high probability that the science target is within the aperture immediately following a slew.

The pixel extraction box size for the standard NIRSpec MSA target acquisition is

~3”x~3”, (or 32 pix x 32 pix) to ensure the target is detected in the setup field. Here we describe a means to tile acquisition images acquired with the 1.”6 square aperture in order to observe a 4x larger region of the sky to center the source for target acquisition. o Step 1: Slew to target position and roll angle. As for standard MSA TA, determine the shift offset position of the grating mirror. Take an internal calibration exposure with the CAA lamp to locate and centroid the position of the fixed slit used for reference. Determine and apply any mirror offset to the known position of the TA and science aperture. o Step 2: Configure NIRSpec for the TA filter and place the object to be centered at the position of the 1.”6 square aperture. The rms of the initial pointing will be ~0.”6.



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SM-12 o Step 3: Construct a 32 x 32 pixel region of the sky centered at the wide aperture position by offsetting and acquiring 3-read MULTIACCUM TA images at each position in a four point offset pattern (Figure 4). Use the three step MULTIACCUM up-the-ramp exposure to reject CRs. Obtain target acquisition images through the wide aperture, one at each of four +/- 0.”8 offset positions, each offset position will have its own associated subarray (as shown in Figure 4): o Slew to the position for offset number 1 and acquire the TA exposure. Clean

CRs and extract a 3.”2 x 3.”2 (32 x 32 pixel) field centered around a subarray position associated with the dither position in the pattern (see Figure 4). The center of the extracted subarray position will change so that the aperture locations map out the wider field on the sky. Save the subarray image.

o Slew to the position for offset number 2 and acquire the TA exposure. Clean out the CRs and extract the 32x32 pixel box centered around this subarray location. Add this subarray image to the one acquired at the previous offset position and save the summed image. Repeat this 2 more times for offset positions number 3 and 4 and their associated subarray centers.

The 32x32 pixel image of the full TA region is constructed by pasting together the subarrays from these four pointings. As presented in Figure 4, the extracted sub-array will be 32x32 pixels for each of the four pointings, but the subarray central position will move with respect to the dither offset so that the 3”x3” field of view on the sky can be reconstructed simply by summing the four pointing subarrays. o Step 4: Execute the standard centroiding algorithm (Valenti, Regan & Balzano

2007) on the constructed 32x32 pixel array. Determine and apply the necessary offset to center the source in the default position of the 1.”6 square aperture.

o Step 5: Take an image of the (now centered) target through the 1.”6 square aperture.

Fine-tune the centering using small offsets to center the target to the optimal value achievable given its S/N (see below).

o Step 6: If necessary, offset the science target from the 1.”6 square aperture to the

IFU or FS aperture used for the science.



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Figure 4: A graphical depiction of the Wide Aperture TA method. A 32x32 pixel image for centering a target is constructed by offsetting through a 4 point dither pattern and extracting and summing subarray images. The black box represents the 1.”6 square aperture, which, of course, does not vary in position.

The middle panels show the proposed offset pattern and subarray location (with respect to the square aperture), and the bottom panel shows the reconstructed 32x32 pixel image of the TA field.

The 1.”6 square wide aperture should be large enough to well sample the extent of the target PSF (e.g., centroiding effects from slit losses will not be an issue), so the accuracy of the centroid calculation of a source within the wide aperture by the onboard scripts will be approximately (Jakobsen 2005b):

Equation 1

" cent

=

$

&

# x

% S / N

'

)

(

2

+ " pix

2

The first term in this expression represents the positional uncertainty of the image center determined from the photon noise and sources of noise in the image itself, the second

!

θ x term encompasses uncertainties in source position arising from read noise, flat fielding noise, and other detector effects. Through detailed optical simulation, the parameters



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θ x

=150mas and σ pix

=5mas have been determined for NIRSpec (Jakobsen 2005b). A value of σ pix

=5mas corresponds to 1/20 th of a NIRSpec pixel, implying that a centroid value better than this limit is not obtainable because of the undersampling of the PSF by the large, 100mas detector pixels. This is consistent with general knowledge on target centering accuracy of undersampled images (J. Anderson, private communication). For faint targets, the

θ x

term in this case will be affected by increased image noise because the 32x32 pixel subarray is formed by summing the TA images instead of pasting them together in FSW. In the low S/N regime (S/N~10 in the summed image), target centroid calculation through the 1.”6 square aperture will have an accuracy on the order of

~15mas. However, for sources with high S/N (S/N>30), the accuracy of the centroid calculation will be limited by the detector pixel size, and hence should approach ~5mas.

The final, total accuracy of centering a target using this method must also factor in an uncertainty of ~5mas associated with a final offset maneuver to center the source in the wide aperture (or FS/IFU see below), the error associated with changing from the TA filter to the science filter (~1 mas), and the estimated error in the assumed aperture location (~6mas; Jakobsen 2005b):

Equation 2

" total

= "

2 cent

+ "

2 slew

+ "

2 filter

+ "

2 aperture

The uncertainty associated with the total centering of a high S/N target ( σ cen t

~5mas) through the 1.”6 square aperture will typically be better than 10 mas.

!

applied to the value necessary for a slew to the FSs or the IFU apertures that will be used for execution of science. The target centering accuracy within the FS apertures is more stringent than the IFU, and the final target centering accuracy using this TA method will likely be limited by the accuracy of the slew offset from the wide aperture to the science position. Offsets on the order of ~20-30” should have average accuracies of better than

~20-30 mas (E. Nelan, private communication). Hence, TA done with this method of centering a source within the 1.”6 square and offsetting into the science aperture should be accurate enough to execute science using faint sources observed using the wide aperture, IFU, 0.”4 fixed slit, and perhaps the 0.”2 fixed slits. The latter application depends on the accuracy of offsetting the target from the square aperture into the 0.”2 FS science aperture .

The TA method described in this section may be particularly useful for wide aperture or fixed slit observations of moving targets. The standard TA method described in section

3 may be difficult to implement for moving targets because the science source will be continuously moving with respect to the MSA reference stars and shutters.

4.3

Fixed-Slit “PEAK-UP”

Offsetting a target by ~20” to ~30” distances should deliver a 1 sigma estimated positioning accuracy of better than ~20-30mas. As mentioned above, merely offsetting by 20-30” to the science aperture after executing the wide aperture TA outlined above may not provide the required centering of the source within the FSs. In this section, we



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SM-12 describe a method for TA for fixed slit observations which does not use reference stars observed through the MSA and is better suited for sources that require more stringent centering constraints than the wide-aperture procedure described in section 4.2 might provide. As such, this TA method may be applicable for science using the 0.”2 fixed slits.

After the initial target acquisition described in section 4.2 (or 4.1) and offset to the fixed slit aperture, the source should be within <30mas (TBD) of the center of the FS used for science. However, higher accuracy centering may be needed, particularly for the 0.”2 wide slits, so a “peak-up” should now be performed. A series of exposures shall be obtained while stepping the telescope in a raster scan across the slit (e.g. Kriss et al.

2002). The raster pattern would optimally be a single line perpendicular to the long axis of the slit, with on order of ~5 offsets. For example and comparison, target peak-up with the narrow 0.”1 wide slit with HST STIS is accomplished with a delivered ~5% accuracy using 5 scan positions (at the initial pointing and offsets of +/- 1.5, 0.75 of the slit width;

0.”075 steps for the 0.”1 STIS slit; STIS Instrument Handbook). Proposed initial raster scan positions for NIRSpec might be [-0.”6, -0.”3, 0, +0.”3,+0.”6] for the 0.”4 FS and [-

0.”3, -0.”15, 0, +0.”15, +0.”3] for the 0.”2 wide FSs. However, the NIRSpec PSF is poorly sampled by the detector pixels and the slits are wider. Hence, optimization of the raster scan positions and pattern frequency is TBD for a comparable peak-up accuracy.

Here we describe the method for executing the fixed-slit “peak-up” after the initial coarse accuracy or wide aperture centering process (Kriss et al. 2002).



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JWST-STScI-001751

SM-12 o Initial Steps: For initial centering setup in the 0.”2 FSs, follow the steps for either coarse accuracy standard TA described in section 5.1, or the procedure for centering a target in the 1.”6 square aperture, described in section 5.2. Position the target at the

FS aperture used for the science (e.g., either of the 0.”2 slits). o Peak-up Step 1: Execute a fixed slit target “peak-up”, acquire images at each step in the 5 point raster pattern: o Obtain three read frames (for CR rejection) at the target peak-up scan location.

o Extract subarray images at the peak-up aperture, reject CRs, and integrate the total intensity. Store the intensity and the scan position. o Move to the next step in the 5 (or 7) point scan pattern and repeat. o Peak-up Step 2: To allow for an accurate centroid calculation, the minimum flux value in the peakup (the PEDESTAL) is subtracted from each step (as done for STIS;

STIS Instrument Handbook). The onboard script then determines the position of maximum flux, using a flux-weighted centroiding technique to determine the optimum flux center to a fraction of a scan position. o Peak-up Step 3: Offset the target to the derived peak-up location. Take a confirmation image of the target through the peak-up aperture to verify the final position.

The target peak-up with STIS takes ~5-10 minutes to execute (STIS Instrument

Handbook). For fainter objects which require longer integration (NRS or NRSSLOW readmode) for target centering with NIRSpec, this process would be correspondingly longer. However, sources that are this faint will likely require pre-imaging to define a target centroid position. Peak-ups would thus not be recommended for the faintest FS targets unless the MSA quadrants fail. The uncertainty associated with FS peak-up depends on the brightness of the target, PSF size, slit width, the number of scan positions used in the peak-up process, and the position accuracy of these small scan offsets. Based on the pixel undersampling of the NIRSpec PSF on the detector and the 0.”2 slit width, a

5% positional accuracy should be achievable for a peak-up using ~5 raster scan positions on a bright source (S/N>30). Further instrument modeling is needed to determine the uncertainties more conclusively (TBD). Following the guidelines outlined in 5.2 on target centering accuracies, the typical uncertainty associated with executing the 0.”2 FS peak-up is:

Equation 3

" total

= "

2 peak # up

+ "

2 slew

+ "

2 filter

+ "

2 aperture where all terms have the same meaning as section 5.2 except for σ peak-up

, which is estimated to be ~10mas for the 0.”2 wide slit. The final accuracy for a coarse centering plus fixed-slit peak up on a bright target is thus better than 15mas, and is constrained by the delivered peak-up accuracy on the target.



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Note that the decision to implement this target acquisition method is pending further information on the accuracy of offsetting sources by ~20-30”. The observatory requirement is for an accuracy of 1% on slews of this size, but the delivered offsetting performance is expected to be much better than this (E. Nelan, private communication).

Fixed slit “peak-up” following centering in the wide aperture may not be needed at all.

5.0

High S/N Bright Object Target Acquisition

A high priority science goal of the NIRSpec instrument is to directly detect the spectral signature of planets during transit events. To do this, very high signal-to-noise spectroscopy (S/N > 10 4 ) must be acquired on extremely bright stars. The 1.”6 x 1.”6 square aperture (as described in section 2 and further discussed in 5.2) will allow GOs to acquire high S/N data on bright stars (AB<8) known to harbor planets. Several of these stars are so bright that observing them using the regular NIRSpec fixed slit subarrays or target acquisition filters will saturate. Special subarrays are defined for the 1.”6 wide aperture which will not saturate the science spectra (Tumlinson 2008).

For initial coarse target centering of extremely bright sources, the TA methods described thus far will not work because the extremely bright science target will saturate when observed in imaging mode through the wide aperture. As a result, we will need a TA procedure that will not saturate the star on the science aperture. This will require target acquisition using dispersed spectral images. Bright object TA using dispersed images has been implemented for COS bright object TA.

In order to define how the bright object, wide aperture TA should be executed, it is important to understand the accuracy necessary for the TA to accomplish the science objectives. In order to achieve S/N~10 4 spectroscopy, the star must be centered well enough in the aperture that very small effects from telescope jitter and tracking drift do not adversely affect the S/N. Figure 5 plots the relative change in wide aperture slit transmission at 2 µ m versus the offset from the wide aperture center (from calculations presented in Jakobsen 2008). The change in slit transmission for the geometrical optics and the transmission including diffraction losses are respectively presented as dashed and solid lines in the plot. In order to stabilize the aperture transmission to better than 2x10 4 , the target centering needs to be accurate to <20mas rms. Plots of the change in slit transmission at other wavelengths yield similar results for the required TA setup accuracy.

One might think that the standard NIRSpec TA process could be altered by a small amount so that TA with <20mas accuracy in the wide aperture can be executed. This would be possible if accurate pre-imaging could be acquired and the first step of the TA process offset the bright target to a location near to the wide aperture but behind the MSA mounting plate (See Figure 6). The full standard TA process could then be carried out, centering the bright star at this fixed offset location to avoid all detector saturation effects. Once the TA corrective slew was determined by the FSW, the grating could be put into the beam and a final slew executed that would offset the star into place at the center of the wide aperture. In practice, however, this method will not be feasible for



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JWST-STScI-001751

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Figure 5: The relative change in wide aperture slit transmission plotted verses the TA setup offset from aperture center.

achieving <20mas TA accuracy. One of the functions of the NIRCam pre-image is to very accurately define the positions of the TA reference stars with respect to the position of the science target. In a full frame NIRCam image, the bright star will saturate very badly and accurate relative coordinates will be difficult to determine. Additionally, this is a particular case where it seems unreasonable to require GOs to use their science allocation time to obtain a NIRCam pre-image for science observations on an extremely bright star.

We will need a method to autonomously acquire a bright star with <20mas accuracy through the 1.”6 square aperture with no pre-imaging. After contemplating several ideas for this (and discussing methods with J. Valenti), it was decided that the best way to execute high S/N bright object TA is to use an offset coarse accuracy standard acquisition followed by a dispersed FS peak-up through the 0.”4 FS. The 0.”4 FS was selected for the dispersed peak-ups because it is the closest FS to the wide aperture. The final offset to the wide aperture will be ~3”, so slew uncertainties in the final target centering will be minimized. The high S/N bright object TA will require a multi-step process:

Coarse Centering: o Step “0” – Instead of slewing to a position that places the science target in the center of the science aperture, start out the coarse centering process by placing the target at a location near to the 0.”4 FS, but behind the MSA mounting plate (Figure 6).



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Figure 6: A proposed offset position nearby to the 0.”4 slit (but behind the MSA mounting plate) for coarse centering of a bright star for high S/N wide aperture planet transit TA. o Step 1 – Step 5 – Execute steps 1 to 5 in the standard NIRSpec TA but using coarse accuracy coordinates as described in section 5.1 to place the target within 100mas of the desired location.

Dispersed Peak-up in the 0.”4 FS: o Step 6: After the coarse accuracy acquisition, the source should be within 0.”1 of the offset location. Place the grating into the NIRSpec beam to avoid saturation in the dispersed peak-up process. Offset the target star to the center of the 0.”4 FS to execute the peak-up. o Step 7: Execute the dispersed peak-up through the 0.”4 FS using 7 scan position locations for high accuracy peak-up (exposures will be very fast in the high S/N regime; offset positions are TBD). Exposures will be acquired in the subarray used for science to avoid saturation. o Obtain one read frame of the target at the peak-up scan location. A three frame MULTIACCUM exposure is not necessary to reject CRs, because the exposures will be very short and the flux of the bright star will be dominated by photon noise.

o Integrate the total intensity in the dispersed subarray image of the bright star.

Store the intensity and the scan position. o Move to the next step in the 7 point scan pattern and repeat. o Step 8: To allow for an accurate centroid calculation as for the regular fixed slit peakups, the minimum flux value in the peakup (the PEDESTAL) is subtracted from each step. The script then finds the position of maximum flux, using a flux-weighted centroiding technique to determine the optimum flux center to a fraction of a scan position. Offset the star to the peak-up location.



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JWST-STScI-001751

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Center in the Cross Dispersion Direction: o Step 9 : Acquire a dispersed subarray image at the peak-up location to centroid the star in the cross-dispersion (or spatial, y) dimension. Sum the flux in the ‘x’ dimension of the subarray to extract a 1-D cross-cut of the flux in the star. Use the standard centroiding algorithm to derive the corrective yoffset slew necessary to place the star at the center of the 0.”4 aperture. The accuracy of this centering in y will go as Equation 1 in the high S/N regime (~5mas).

Final Slew: o Step 10: Add the yoffset corrective slew to the value necessary to place the bright star at the center of the wide aperture. Execute this final slew.

The final uncertainty associated with this TA method is:

Equation 4

" total

= "

2 peak # up

+ "

2 y _ off

+ "

2 slew

+ "

2 filter

+ "

2 aperture

Where σ peak-up is the uncertainty in the 7 position dispersed peak-up scan. This is the dominant component and needs to be less than 18mas to deliver a total accuracy of better

! scan position locations with the 0.”4 wide slit. Further instrument modeling is required to estimate the optimal scan position step size and delivered accuracy for the dispersed peak-ups (TBD).

It is noted that a modified standard TA with coarse centering followed by a dispersed peak-up will require a larger time overhead for TA. However, the method described here will give the best accuracy for bright objects observed through the wide aperture.

Additionally, implementing this method for TA will ultimately save on-sky observing time because prime science allocations need not be used for pre-imaging of very bright stars.

6.0

Recommendations and Priorities for Implementation

The five methods for NIRSpec TA described in this report are summarized in Table 6-1.

Included in the table are the observing strategies that the indicated TA is useful for, the delivered accuracy, an estimate of the time duration compared to the standard method, and the estimated complexity and priority for implementing the TA process. The standard NIRSpec TA method is included here, along with the four new proposed TA acquisition strategies. The four new methods described here do not require NIRCam preimaging that would result in costly Phase II deadlines for all NIRSpec programs.

Additionally, implementation of new capabilities will be necessary for acquiring

NIRSpec targets in the high S/N bright object science case, and in the event of MSA quadrant failure or problems with the standard TA process.



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TA Method

Table 6-1 Summary Table of NIRSpec TA Strategies

JWST-STScI-001751

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Reference

Section

Observing

Strategy

Accuracy TA Time

Duration

Implementatio n

Priority

Standard 3.0 MSA/ Crowded

Field / PI Request

<20mas ~600s

(depends on reference star brightness)

<100mas ~600s or less

In progress highest

Modified

Standard

(coarse accuracy)

High S/N

Bright

Object

4.1 IFU / Wide aperture faint object / Moving

Target

Wide Aperture bright source

Slight modification to standard high

Wide

Aperture

Centering

Coarse

Centering +

FS Peak-up

5.0

4.2

4.3

IFU / Wide

Aperture / FS

(MSA Failure)

FS TA (0.”2 slit)

<20mas

<20mas

< 5% of slit width longer than

600s

Depends on target brightness

Depends on target brightness

Includes modified standard TA and new capabilities

New capabilities in

FSW

New capabilities in

FSW high (bright object science)

Medium/high

(for commissioning or MSA failure)

Lower (pending info on target offset accuracies)

The standard NIRSpec TA procedure is already being implemented in the commanding scripts. The modified standard TA with coarse centering accuracy requires the smallest amount of adaptation to the existing TA scripts, and thus could be implemented with little impact on schedules without pre-imaging TA on IFU, wide aperture and moving targets.

The other three TA methods will require effort to implement, either by small additions to the existing TA or with completely new script capabilities. The final priorities for implementation of new TA methods must be discussed and set by the NIRSpec

Operations Working Group, and will likely be driven by science and scheduling constraints.

The high S/N bright object TA requires new functionality in the TA scripts, but will be the primary means to acquire targets for the high profile planet transit science case. As such, this TA is recommended for implementation prior to launch (consistent with the high S/N wide aperture science priority).

If the standard TA procedure fails during science operations or in the NIRSpec commissioning phase, centering a target through the wide aperture and offsetting will be the primary means to acquire FS and IFU targets. Existence of this back-up TA method will be particularly important during instrument commissioning for tests with the IFU and

FSs when initial problems or difficulties with the standard TA procedure are being worked out. During commissioning, it will also be important to characterize the TA centering and offsetting accuracies that are achievable with NIRSpec, and verify how useful the wide-aperture centering TA method will be in the event of MSA quadrant



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SM-12 failures. For this reason, implementation of this TA capability is recommended prior to

NIRSpec instrument commissioning.

As noted in section 4.3, fixed slit peak-ups with the 0.”2 FS following a coarse accuracy or wide aperture centering may not be needed. Peak-ups would not be necessary if wide aperture centering and offsets to the 0.”2 slit can be executed with an accuracy of better than 20mas. We do not yet request implementation of FS imaging peak-ups in the scripts, pending more information on slew accuracy performance. Discussion is included in this report as a place-holder to note that this may be required for autonomous acquisition with the 0.”2 FSs.

7.0

References

Chayer, P. et al. 2008 “Extrapolating the Properties of the Guide Star Catalog to the Near

Infrared”, [JWST-STScI-000920]

Jacobsen, P. 2008 Proposal for a Wide Aperture in NIRSpec, presentation made to

NIRSpec IST

Jacobsen, P. 2007 “Observing Eclipsing Exoplanets with NIRSpec”, private communication

Jacobsen, P. 2005b “Error Budget for the NIRSpec Target Acquisition”, [JWST-REF-

005938]

Jacobsen, P. 2005a“The NIRSpec Target Acquisition Concept”, [ESA-JWST-TN-560]

Kriss, J. et al. 2001 “Recommendations for NGST Target Acquisition and Peak-Up

Requirements” [JWST-STSCI-000310]

McCallon, H. L. 2002 http://web.ipac.caltech.edu/staff/hlm/2mass/overv/overv.html

Pier, J. et al. 2003 Astronomical Journal, 125, 1559

Tumlinson, J., “NIRSpec Subarrays for Planetary Transits and other Bright Targets”,

2008-10-27 [JWST-STScI-001601]

Valenti, J. et al. 2006 “Properties of the Software Tool Used to Define Observations with the NIRSpec Micro-Shutter Array (MSA)”, [JWST-STScI-000758]



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