NIRSpec Wide Aperture Target Acquisition
The JWST NIRSpec wide aperture target acquisition (WATA) observing sequence performs target acquisition of stationary and moving science targets, as well as faint stationary offset targets using the S1600A1 aperture.
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Unlike the standard target acquisition method (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. WATA offset target acquisitions may yield poorer final pointing accuracy due to any additional uncertainty in the relative coordinates of the offset target and final science target.
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 ~ 11.9 mag < AB < ~25.7 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 ~ 11.9 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 ~ 11.9 mag is not presently possible with 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 (1) use the standard MSATA algorithm which utilizes faint reference stars in the field, (2) acquire a nearby fainter star and offset to the desired bright target (fixed targets only), or (3) observe without using any target acquisition.
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. A one-sigma slew translational uncertainty is 7 mas. 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) is the default method used to acquire most targets for high S/N spectrophotometric observations of bright object time-series (BOTS) targets, including exoplanet transits in the brightness range 11.2 < J Mag < 25.0 (or alternatively, 11.9 < AB Mag < 25.7).
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 the images acquired for accurate target acquisition in BOTS science mode. 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 performance during ground testing. These values are subject to change when on-sky performance and sensitivity is verified 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).