Following a slew of the JWST spacecraft, the JWST on-board software can execute target acquisition (TA) corrections that are designed to remove any initial pointing errors - i.e., inaccuracies in the absolute guide star and science target positions. The purpose of the NIRSpec TA procedures is to fine-tune the JWST pointing to very accurately place a given target into one of the fixed slits, the IFU, or to precisely align a set of targets within the grid of MSA shutters.
Two methods have been designed for NIRSpec target acquisition: standard TA using the MSA (called TACQ), and the bright object TA (BOTA) through the wide FS aperture. Like all JWST observations, the NIRSpec standard target acquisition is executed autonomously. A third option, called "VERIFY_ONLY", is available in some observing modes to only assess the field pointing without executing a target acquisition sequence or corrective small angle maneuvers of the telescope pointing.
Available methods for NIRSpec target acquisition
NIRSpec observers can choose between three available methods for target acquisition methods: the standard method (called TACQ), the Bright Object TA (BOTA) method and the VERIFY_ONLY option. Because different NIRSpec science modes require different degrees of pointing accuracy, not all methods are available in all modes. Table 1 shows the availability of TA methods vs. the science observing mode.
Table 1. Target acquisition methods available for each NIRSpec observing mode.
The standard TA option, TACQ uses reference stars observed through the open MOS shutters in the MSA to accurately align science sources with their dedicated apertures. The following table describes the various TA methods, their expected performances, and the methods used to compute the corrective small-angle maneuver.
Table 2. NIRSpec Target acquisition methods with accuracy and brief descriptions.
|TA Type||TA Approximate Accuracy||Description|
|Standard target acquisition (TACQ)|
Expected accuracies of approximately 20 - 60 milli-arcseconds (Depends on relative astrometric accuracy of reference targets and science sources).
|TACQ will be used for most NIRSpec science observations performed with the micro shutter assembly (MSA). The TA reference objects are observed through the open micro-shutters, and centroids are calculated and used to accurately correct the initial pointing and orient for all NIRSpec observations that use TACQ. 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 TACQ procedure is designed to work with input relative astrometric accuracy of 5 to 50 milli-arcseconds. Relative astrometric accuracy of 5 mas in the TA and science planning catalog is necessary to deliver a TA accuracy of 20 milli-arcseconds. 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. |
|VERIFY_ONLY||JWST Pointing Performance - 0".68, 1sigma radial.||The NIRSpec VERIFY_ONLY method 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. VERIFY_ONLY can also be used for placing moving targets within the IFU aperture. Presently, this option is only available in IFU and MOS science modes, the expected accuracy can be too poor for the narrow FSs. |
|Bright object target acquisition (BOTA)||Expected accuracy of target placement in the NIRSpec Wide Aperture to 20 milli-arcseconds (Depends primarily on centroiding accuracy of the target).||The NIRSpec bright object target acquisition (BOTA) is the default method used to acquire most targets for high S/N spectrophotometric observations in bright object time series (BOTS) mode, including transiting exoplanets in the brightness range 10.0 < K < 16.1 (or, alternatively, 11.9 < AB < 18.1). This method corrects any errors in absolute telescope pointing (and absolute knowledge of the source coordinates) by executing a TA centroid of the science target and offsetting it to the center of the wide aperture. |