NIRSpec Target Position
The JWST NIRSpec VERIFY_ONLY target acquisition option takes a pointing verification image acquired through the MSA shutters and the open IFU aperture.
The NIRSpec instrument provides an option to verify the position of a target, but not do any corrective acquisition activities.
The NIRSpec VERIFY_ONLY1 target positioning check can be used for MOS and IFU observing modes. Although it is presented here as a target acquisition (TA) method, this option does not do any corrective acquisition activities. It only acquires an image through the all open or configured MSA shutters and IFU open aperture. This is so that the precise pointing can be determined by analyzing positions of unsaturated stars, after the observation executes. The raw performance of the VERIFY_ONLY target positioning method depends on the guide star accuracy and user knowledge of the science source coordinates. This method is very useful for science in the IFU mode, where the 3" × 3" square field of view is forgiving of uncertainties in the science target position (see JWST pointing performance information). This method can also be used for MOS Mode science for very extended targets that are larger in size than a single 0.2" MSA shutter width.
The VERIFY_ONLY and WATA options are now available for observations of moving solar system targets in the NIRSpec science modes (IFU and MOS).
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Operational details of the VERIFY_ONLY method
This method is called VERIFY_ONLY. A visit that uses this option consists of four main steps:
- Slew the telescope to the desired field.
- The FGS acquires a guide star and is set to fine guiding.
- Execute NIRSpec science exposures in MOS or IFU observing modes.
- Obtain a "pointing verification" (PV) image through the micro-shutter assembly (MSA) with the shutters configured to "ALLOPEN" or with an optional blocking MSA shutter configuration. An "ALLCLOSED" MSA shutter configuration can be used for fields that have very bright sources or high nebulosity. The IFU aperture is also open to the sky, so the IFU field of view will also be imaged on the detector.
Unlike the MSA TA method (MSATA), or the Wide Aperture TA (WATA), the NIRSpec VERIFY_ONLY option does not execute any target acquisition activities. Visits that use the VERIFY_ONLY method execute all NIRSpec science exposures in a visit, and then acquire a "pointing verification image" (PV image) through the MSA shutters configured all open to check the field pointing. The PV image is taken after the spectroscopy is completed. By default, this image will be acquired with the grating wheel assembly (GWA) set to MIRROR with full frame detector readout (no subarrays are allowed). The PV image is acquired using one of the three TA filters (F110W, F140X, or CLEAR) in the filter wheel assembly (FWA). Note that selecting VERIFY_ONLY (or NONE) will limit the options for the collection of confirmation images for MOS observations. To avoid affecting subsequent science exposures with detector persistence, it is possible for users to supply a custom MSA open/closed shutter configuration that blocks out bright objects that would saturate, while still imaging fainter sources to obtain a good pointing verification solution. However, an MSA configuration other than ALLOPEN or ALLCLOSED will require an aperture position angle assignment entailing a planning process similar to MOS observation planning. That process requires the use of MPT to design an MSA configuration at the last pointing of every visit. For this reason, and because of increased demand on the scheduling system, the use of protected MSA configurations should be a last resort.
Accuracy and approximate brightness limits for VERIFY_ONLY
The positioning accuracy of the VERIFY_ONLY PV image is expected to be limited by the JWST absolute pointing accuracy. The PV image obtained with NIRSpec can be analyzed after the observation to provide knowledge of the exact pointing. The image can be aligned with, or referenced to, other catalogs that have better astrometric accuracy in the observed field. The pointing precision that can be derived from the image will depend on the quality of the alignment catalog and the number and distribution of the comparison sources in the field. Users should also note that the MSA shutter bar support structure causes vignetting of the imaging field. Both stars and extended objects will have centroid effects from these shutter bars. No particular verification performance can be promised or guaranteed, though the assumption is that post-observation user analysis can correct the pointing knowledge to better than the JWST observatory pointing accuracy.
The observation brightness limits for the VERIFY_ONLY target position option are essentially the same as sensitivity limits for NIRSpec in imaging mode with the GWA set to MIRROR. The most up-to-date sensitivity and accurate signal-to-noise estimates for a range of target source spectral distributions can be derived in the JWST Exposure Time Calculator (ETC) using the NIRSpec target acquisition calculation option for "single object with the MSA".
Brightness estimates to gauge sensitivities for VERIFY_ONLY pointing verification images can be obtained using the JWST ETC in the NIRSpec target acquisition calculation option. The "MSATA (single object)" calculations in the instrument setup area should be used. Right now these ETC calculations do not accept exposure parameters—the presented sensitivities for S/N are for Ngroups = 3. Sensitivity in the images can be varied by selecting the NIRSpec detector readout pattern, and the imaging FILTER used for the observation (F110W, F140X or CLEAR).
Planning options for VERIFY_ONLY
If the VERIFY_ONLY option is used, the IFU or MOS science observations will be defined in a standard way via the Astronomer's Proposal Tool (APT). The RA and Dec of the pointing verification image will be the position of the telescope at the end of the science observation execution (i.e., the last dither position). For the VERIFY_ONLY method, users must select the filter for the pointing verification image, the parameters necessary to specify the exposure time in the image (Readout Pattern and Ngroups), and the MSA shutter configuration used for the image.
The MSA quadrant fields-of-view for VERIFY_ONLY pointing verification images in IFU or MOS observations can be seen in the APT Aladin image interface by clicking the FoV (field-of-view) button.