NIRSpec Verify Only Target Acquisition

The JWST NIRSpec VERIFY_ONLY target acquisition option takes a pointing verification image acquired through the MSA shutters and the open IFU aperture, but does not update the initial pointing set by the guide star acquisition.

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See Also: NIRSpec Target Acquisition Recommended Strategies

The NIRSpec instrument provides an option to verify the position of a target, but not do any corrective acquisition activities.

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The NIRSpec VERIFY_ONLY 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 NIRSpec micro-shutter array with either the ALLOPEN or ALLCLOSED MSA shutter configuration and the IFU aperture open. This is so that the precise pointing can be determined by analyzing positions of unsaturated stars seen through the MSA, 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 reference star and science target source coordinates. This method may be very useful for science in the IFU mode, depending on the source extent and the dither separations, where the 3" × 3" square field of view is forgiving of uncertainties in the science target position (see JWST pointing performance information). This method may also be useful for MOS mode science observations of extended targets that are larger in size than a single 0.2" MSA shutter width. 

In the undispersed light, the MSA quadrants and the IFU slices project onto different locations on the NIRSpec detectors and in principle this can allow an image of the IFU field of view as seen through the selected target acquisition filter to be reconstructed and precisely aligned relative to field objects imaged through the MSA. However, there are currently no tools to support such image IFU reconstruction and alignment, and in many cases collapsing the dispersed IFU science observations over wavelength to produce an image will provide similar information in a more easily used form.

The VERIFY_ONLY and WATA options are now available for observations of moving Solar System targets in the NIRSpec science modes (IFU and MOS).

Operational details of the VERIFY_ONLY method

This method is called VERIFY_ONLY. A visit that uses this option consists of 4 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. Alternatively, 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. Because the use of a custom MSA configuration to block only the bright stars would place additional constraints on orientation and scheduling, the use of such a custom MSA configuration is not currently supported for VERIFY_ONLY images.

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) at the spacecraft pointing of the final science exposure.  Please note, however, that an image of the target in the science filter can be constructed from an IFU data cube itself, so verification imaging may not be necessary for IFU observations. If neither MSA nor IFU verification imaging is needed, be sure to select TA Method = "None" to omit this imaging.

The PV image is taken after the spectroscopy is completed in a visit. 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 3 TA filters (F110W, F140X, or CLEAR) in the filter wheel assembly (FWA).

Note that selecting VERIFY_ONLY (or NONE) may limit the options for the collection of confirmation images or parallels when it is used with MOS observations

Images taken with the ALLOPEN MSA configuration when there are bright stars in the field of view may result in significant saturation, which can cause persistence in subsequent images. However, since VERIFY_ONLY images are only taken at the end of each visit, this will be of concern only if there will be additional NIRSpec visits in the following hour or so after a verification image. This can occur in a single observation which was split into several visits to support multiple pointings that exceed the visit splitting distance, or for multiple observations grouped as a non-interruptible sequence. In such cases, it might be desirable to use the ALLCLOSED MSA configuration, although the observer will need to reconstruct and interpret the image as seen through the IFU for the verification image to be useful.

Note also that verification imaging while tracking on a moving target with the MSA ALLOPEN is not recommended.  It will typically cause trailing of fixed background sources in the MSA. APT will include checks for this and other conditions and will warn the user when needed.

The images taken using VERIFY_ONLY differ from the optional confirmation images that can be selected with MOS observations. The confirmation images are taken using the filter used for the immediately-following spectroscopic exposure and also use the MSA configuration that will be in place for that exposure.

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. Centroid measurements of both stars and extended objects will be impacted by 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.  Signal-to-noise ratio estimates can be derived in the JWST Exposure Time Calculator (ETC) using the NIRSpec "MOS Verification Imaging" or "IFU Verification Imaging" calculations, depending on whether the signal-to-noise ratio in the IFU aperture or on the MSA is desired.

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.

Notable updates
    Noted that Verify_Only selection in APT now has appropriate warnings.

    Added clarification that the use of custom MSA configs is not currently supported; added some additional information clarifying saturation risks
Originally published