NIRSpec Target Acquisition

JWST NIRSpec observing modes utilize target acquisition procedures to place targets in their respective apertures.

See Also: NIRSpec Target Acquisition Recommended Strategies

Following guide star acquisition and a spacecraft slew, JWST's onboard software can execute target acquisition (TA) corrections that are designed to remove any initial pointing errors. These might be due to inaccuracies in the absolute guide star and science target positions, for instance. NIRSpec TA procedures are used to fine-tune JWST pointing to very accurately place a target in one of the fixed slits (FS), the IFU, or to precisely align a set of targets within the grid of micro-shutter assembly (MSA) shutters.

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Four options are provided for NIRSpec target acquisition (see Table 1 for availability by instrument mode): MSATA, WATA, VERIFY_ONLY, and NONE. The options VERIFY_ONLY and NONE forego TA and fine pointing correction: VERIFY_ONLY will acquire imaging to assess the field pointing in post-observation analysis, and NONE will skip both TA and TA verification imaging.

This article provides a brief overview of the available methods. For more detailed information about choosing and implementing the best possible TA, see NIRSpec Target Acquisition Recommended Strategies.

Available methods for NIRSpec target acquisition

See also: NIRSpec Target Acquisition Recommended Strategies

NIRSpec observers have 4 options for target acquisition. The MSA TA method (called MSATA) uses MSA reference stars. The wide aperture TA method (WATA) uses the the 1.6" square aperture (S1600A1). The remaining 2 options are no-TA options: VERIFY_ONLYand NONE. Because different NIRSpec science modes require different degrees of pointing accuracy, the range of available options varies among modes. Table 1 shows the availability of TA options for each science observing mode.

Table 1. Target acquisition methods available for each NIRSpec observing mode

Target acquisition option
NIRSpec observing modes or methods
MOSIFUFSBOTSMoving targets




* MSATA, uses reference stars observed through the open MOS shutters in the MSA to accurately align science sources with their dedicated apertures.

WATA and NONE are available in the MOS APT template in the observation specification form for observations planned outside the normal MPT planning process. This process is primarily intended for long slit observations of extended or moving targets, but may be applicable to other science targets as well.

Using no TA (i.e. TA=NONE or VERIFY_ONLY) will result in the blind pointing accuracy of JWST, which is half of the width of MOS shutters and the 0.2" fixed slits.  FS mode science with TA=NONE is only recommended if mosaic tools are used to create FS scan observations.

A note about terminology:

Guide star acquisition with a guide star in the Fine Guidance Sensor is different from target acquisition which this article describes. Guide star acquisition occurs at the start of each visit, delivering an expected fine pointing accuracy of 0.10" (1-σ error, per axis). Following that, either the MSATA or WATA target acquisition method can be used to place the science source into a small science aperture with the accuracy required to perform subsequent science observations.

MSATA, described in Table 2, is the preferred target acquisition method for multi-object spectroscopy with the MSA. It uses reference stars in the MSA FOV, which are different from the single guide star used to more coarsely point the telescope during guide star acquisition.

WATA, also described in Table 2, is an alternative target acquisition method. It can be performed with the science target itself, or optionally, with an offset starA nearby offset star is used, for example, when the science source is too bright for WATA itself, and could saturate in the first group of the detector readout pattern.

Table 2 describes the various TA methods, their expected performance, and the methods used to compute the corrective small angle maneuver.  For more guidance on TA methods, see NIRSpec Target Acquisition Recommended Strategies.

Table 2. NIRSpec target acquisition methods with accuracy and brief descriptions
TA typeTA approximate accuracyDescription
MSA target acquisition (MSATA)

Expected accuracies of approximately 20–60 mas (depends on relative astrometric accuracy of reference targets and science sources)

MSATA will be used for most NIRSpec science observations performed with the micro-shutter assembly (MSA). The MSATA reference stars, provided in the observer's input catalog via the MSA Planning Tool, are observed through the open micro-shutters. Their centroids are calculated and used to accurately correct the initial pointing and orientation for all NIRSpec observations that use MSATA. The delivered TA performance depends significantly on the astrometric accuracy of both the reference stars and the science sources in the catalog used to plan the TA and the science. The MSATA procedure is designed to work with input relative astrometric accuracy of 5 to 50 mas. A relative astrometric accuracy of 5 mas in the TA and science planning catalog is necessary to deliver an MSATA accuracy of 20 mas. This is the best estimated accuracy achievable for placing science targets into the narrowest NIRSpec apertures: the 0.2" wide NIRSpec MSA slitlets and the 0.2" NIRSpec fixed slits. Guidance on how to select MSATA reference stars can be found in the article on MSATA Reference Star Selection Recommended Strategies.
Wide aperture target acquisition (WATA)Expected accuracy of target placement in the NIRSpec wide aperture to 20 mas (depends primarily on centroiding accuracy of the target).WATA is available for observing with the IFU, the fixed slits, and for multi-object spectroscopy (MOS) planned without the use of the MSA Planning Tool (MPT). WATA is the default method to acquire most targets for the Fixed Slit (FS), IFU, and  bright object time-series (BOTS) modes. This method corrects any errors in absolute telescope pointing (and absolute knowledge of the source coordinates) by centroiding the acquisition target (which may not be the science target) in the S1600A1 aperture and then calculating the slew to move the science target directly to the desired science aperture position for the first science observation.
VERIFY_ONLYJWST Pointing Performance

The VERIFY_ONLY option, available only for IFU and MOS observations, relies only on guide star acquisition performed by the FGS to establish the telescope pointing. At the end of a visit, a verification image is taken with the MSA ALLOPEN configuration. Alternatively, a protective MSA configuration can be used to prevent persistence in subsequent science exposures. 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. It should also be noted that for the IFU, an image can be reconstructed from the data cube itself.

The verification image can be used in post-observation analysis to assess the pointing accuracy, especially for moderately extended or asymmetric targets where WATA centroiding would not work. This method is useful for IFU observations that can tolerate some positional uncertainty (compact targets and small IFU dithers), or for extended objects observed with a long slit in the MSA. VERIFY_ONLY may be used to check the pointing on moving target observations, but will typically cause trailing of sources in the MSA field of view while JWST tracks on the moving target. Note that protected MSA configurations for VERIFY_ONLY TA with moving targets are not permitted at this time.

NONEJWST Pointing PerformanceThe NONE option relies only on guide star acquisition performed by the FGS to establish the telescope pointing. This method is ideal for placing compact sources in the IFU aperture, or when using the MSA to place a long slit on extended objects. NONE can also be used for placing moving targets within the IFU aperture. This option is available for all science modes; however, the expected pointing accuracy may be inadequate for the narrow FSs or the MSA slits, so NONE should only be considered for use with moderately extended sources for those modes.

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