NIRSpec Wide Aperture Target Acquisition
The JWST NIRSpec wide aperture target acquisition (WATA) observing sequence performs target acquisition of single stationary and moving science targets, as well as faint stationary offset targets using the S1600A1 aperture.
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Unlike MSATA, WATA acquisitions are not tied to a particular orientation and so WATA will normally be the acquisition method of choice for single point source targets that have no specific orientation requirement. Required input target coordinate accuracy is 100 mas or less, and for cases not involving offset targets, WATA should yield final pointing accuracy on the order of 20 mas. When directly acquiring the science target, the WATA acquisition procedure can often compensate for minor inaccuracies in the target's coordinates and still yield an accurate centering of the target in the aperture; however, when using an offset acquisition target, any inconsistency between the coordinates or proper motions of the science and offset targets will directly translate into an error in the final target position in the aperture plane.
As WATA was designed to be used with point source targets, if a source is extended, accuracy may be degraded, and in any case an extended source would need to have a sufficiently point-like central concentration with suitable contrast to allow the algorithm to succeed; the algorithm is attempting to find the flux-weighted center of the brightest 0.3” × 0.3” region. The adopted brightness and coordinates should refer to that central concentration. Caution should be exercised when using catalog magnitudes for extended objects, as they may include the flux over a much larger region.
For fixed targets, the principal investigator of a proposal is responsible for ensuring that supplied target coordinates, parallax, proper motions, and the epoch for the position are sufficiently accurate for the science targets as well as any offset targets. For moving targets, the correct orbital elements must be supplied. Observers are also responsible for ensuring that targets used for WATA are of suitable brightness for the selected filter and readout pattern, and are sufficiently point-like to be accurately centroided by the WATA operational sequence. Observations that fail due to inaccurate coordinates, parallax, proper motions, target brightness, geometry, orbital elements, or use of the incorrect epoch for the position will not be repeated.
WATA operational sequence
The NIRSpec wide aperture target acquisition (WATA) method is used to acquire suitably bright objects. It may be used with all templates for situations when roll correction is not critical (i.e., when the orientation achieved through blind pointing is sufficient). Following the guide star acquisition, the operational sequence of WATA is:
- The telescope is slewed to place the acquisition target in the S1600A1 aperture.
- An Ngroups = 3 image covering 32 × 32 pixels, centered at the location of the S1600A1 wide aperture, is acquired.
This exposure is taken using the NIRSpec imaging mirror in the grating wheel assembly. Note that the flux sensitivity of the image is set by (1) the TA filter selected for the WATA observation and (2) the exposure time that is defined by the selected detector readout pattern and subarray. All NIRSpec TA images are acquired with Ngroups = 3 (Acq Groups/Int parameter in APT).
- Cosmic ray and hot pixel amelioration is performed on the image.
- The star's centroid is calculated and pixel coordinates are transformed to a sky coordinate system.
- The telescope performs a small angle maneuver to center the star in the S1600A1 aperture.
- A mandatory post-target acquisition "reference image" of the centered TA object is obtained as part of the procedure. This exposure must be obtained in the same visit as all of the science exposures that depend on the acquisition.
If it is necessary to split an observation into multiple visits, additional observations and acquisitions will be needed. Any slew needed between the acquisition pointing and all subsequent science pointings must fit within the visit splitting distance and there must be at least one guide star that can support all the needed pointings. This will limit the size of mosaics and other offsets that can be supported following the WATA acquisition.
The operational sequence of activities carried out by WATA is hard-coded in the NIRSpec observation execution onboard scripts. The only user-selectable parameters are the subarray type, filter, and detector readout pattern for the observation:
- Three subarrays, which correspond to 3 different readout times, are available: SUB32, SUB2048, and FULL.
- Two readout patterns are available: NRSRAPID and NRSRAPIDD6. The choice will depend on target brightness and achievable S/N: NRSRAPIDD6 is meant for fainter targets that need longer exposure time for the S/N in TA. The NRSRAPIDD6 readout option observes a longer exposure and so it takes a bit longer to execute. The exposure timing for NRSRAPIDD6 and NRSRAPID with different TA subarray options will be reflected in the ETC results. These readout patterns are described in NIRSpec Detector Readout Modes and Patterns.
- Three NIRSpec filters are available: F110W, F140X, and CLEAR. These are expected to allow coverage of the brightness range ~ 12.8 mag < AB < ~26.2 mag.
Table 1. Readout time options
|Subarray||Size (pixels)||NRSRAPID readout time (s)||NRSRAPIDD6 readout time (s)|
|FULL||2048 × 2048||10.737||75.158|
|SUB2048||2048 × 32||0.902||6.314|
|SUB32||32 × 32||0.015||0.105|
Note on saturation: for targets brighter than AB ~ 12.8 mag, saturation occurs in the standard 3-group (Ngroups = 3) TA images obtained, even with the least sensitive WATA acquisition configuration. As a result, direct acquisition of targets brighter than AB ~ 12.8 mag will result in degraded centering when using WATA due to the impact of saturation on the 3-group pixel-by-pixel minimization algorithm employed in the centroiding procedure. Users who wish to observe brighter targets will need to either:
- Use the standard MSATA algorithm which utilizes faint reference stars in the field,
- Acquire a nearby fainter star and offset to the desired bright target (fixed targets only),
- Observe without using any target acquisition, or
- Perform WATA but accept the risk of degraded centering accuracy.
If only the central pixel is saturated, pointing accuracy should still be good to better than one pixel or 0.1", and would be expected to offer some improvement on the expected initial pointing accuracy without TA. If the predicted saturation only occurs late in the third group, performance may be close to nominal.
Note on very faint targets: The current TA algorithm works by taking differences between adjacent groups with negative numbers clipped to zero, but does not currently include any correction for drifts in the detector pedestal level between groups. If the pedestal level drops between adjacent groups, this can cause targets with too few counts in the central pixel to become invisible to the TA algorithm, resulting in a failed target acquisition. This uncertainty is accounted for in the ETC signal-to-noise calculation, and should not be an issue if the source meets the required S/N of 20 or better, but if there is any doubt about whether the target brightness meets this standard, a more sensitive filter and readout combination should be considered.
WATA method accuracy
The NIRSpec wide aperture target acquisition (WATA) method corrects any errors in absolute telescope pointing and astrometric source accuracy by executing a TA centroid on the science source, then offsetting it to the center of the wide aperture.
The required input target coordinate accuracy for WATA is 100 mas or better. The expected accuracy of target placement in the NIRSpec wide aperture (S1600A1) should be 20 mas (1/5th of a pixel) or better for a S/N = 20 target image. The accuracy depends primarily on centroiding accuracy of the target. Typical translational uncertainty for dithers and slews between apertures should be 2–4 mas rms (1-σ per axis) on the sky. The article NIRSpec Target Acquisition Recommended Strategies should be consulted for specific recommendations and considerations using WATA.
WATA brightness limits
The NIRSpec wide aperture target acquisition (WATA) can be used to acquire point source targets in the brightness range 11.9 < J Mag < 25.3 (or alternatively, 12.8 < AB Mag < 26.2).
The JWST NIRSpec instrument uses all reflective optics and has a very high system throughput. Table 2 presents the approximate J-band magnitude ranges for targets acquired using WATA as a function of the selected readout pattern, subarray, and filter. These brightness ranges for WATA exposures are set by the 3-group saturation limit on the bright side, and the signal-to-noise ratio (SNR) = 20 limit on the faint side. The most accurate target acquisition is expected to require images with a SNR of greater than 20.
Table 2. Approximate J-band Vega mag (blue) and AB mag (red) ranges for SNR = 20 to saturation for WATA
|Readout mode in subarray||F110W||F140X||CLEAR|
|SNR = 20||Saturation||SNR = 20||Saturation||SNR = 20||Saturation|
SUB32 (32 × 32 pixels)
SUB2048 (2048 × 32 pixels)
FULL (2048 × 2048 pixels)
Table note: All presented brightness limits are based on best-known information from instrument on-sky performance and sensitivity during JWST NIRSpec instrument commissioning.
Brightness estimates for WATA can be obtained using the JWST ETC in the NIRSpec target acquisition calculation option using the WATA calculations (in the instrument setup area).