The target acquisition procedures used to point the telescope with different JWST NIRSpec observing modes are addressed.
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.
Four methods have been designed for NIRSpec target acquisition (see Table 1): standard TA using the MSA (called MSATA), and the Wide Aperture TA (WATA) through the wide FS aperture. Like all JWST observations, the NIRSpec standard target acquisition is executed autonomously. Two other options, called "VERIFY_ONLY", and "NONE" are available in some observing modes to forego TA and fine-pointing correction. The first of these (VERIFY_ONLY) will acquire imaging to provide a means to assess the field pointing in post-analysis, and the second option will not only skip TA, but also the TA verification imaging altogether.
Available methods for NIRSpec target acquisition
NIRSpec observers can choose between four options for target acquisition: the standard method (called MSATA), the Wide Aperture TA (WATA) method, the VERIFY_ONLY option, and an option called NONE. Because different NIRSpec science modes require different degrees of pointing accuracy, the range of available choices varies among modes. Table 1 shows the availability of TA methods or 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|
|NIRSpec Standard TA (MSATA)|
|NIRSpec Wide Aperture TA (WATA)|
The standard TA option, MSATA, 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 Note 1: WATA and NONE are available from the MOS APT template at the Observation level for moving target observations planned outside the normal MPT planning process.
Table 2. NIRSpec Target acquisition methods with accuracy and brief descriptions.
|TA Type||TA Approximate Accuracy||Description|
|Standard target acquisition (MSATA)|
Expected accuracies of approximately 20 - 60 milli-arcseconds (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 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 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 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".45, 1 sigma, 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. At the end of a Visit, a Verification image is taken with the MSA ALLOPEN or in a protected MSA configuration. This image can be used in post-analysis to assess the pointing accuracy.|
|Wide aperture target acquisition (WATA)||Expected accuracy of target placement in the NIRSpec Wide Aperture to 20 milli-arcseconds (Depends primarily on centroiding accuracy of the target).||The NIRSpec Wide Aperture Target Acquisition (WATA) 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 (Vega Mag) 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. WATA is also available for observing with the IFU, the Fixed Slits, and for MOS planned without the use of the MSA Planning Tool (MPT). In these cases, after centroiding the target in the wide aperture, and offset is applied to move to the aperture that will be used for science.|
|NONE||JWST Pointing Performance - 0".45, 1 sigma, radial.||The 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 in all science modes, but because the expected accuracy can be poor for the narrow FSs or the MSA slits, it should only be considered for use with moderately extended sources in those modes.|
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