The target acquisition procedures used to point the telescope with different JWST NIRSpec observing modes are addressed.
Parent article: NIRSpec Operations
Following guide star acquisition and 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 (FS), the IFU, or to precisely align a set of targets within the grid of MSA shutters.
Four options are provided for NIRSpec target acquisition (see Table 1 for availability by instrument mode): MSATA1, WATA, VERIFY_ONLY, and NONE. The options "VERIFY_ONLY", and "NONE" 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.
This page 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.
1 Bold italics font style indicates parameters, parameter values, and special requirements that are set in the APT GUI.
Available methods for NIRSpec target acquisition
NIRSpec observers can choose between 4 options for target acquisition. The standard TA method (called MSATA) uses MSA reference stars. The wide aperture TA method (WATA) uses the wide fixed slit aperture. The remaining two options are no-TA options: VERIFY_ONLY and 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 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.
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 pointing accuracy of 0.45" (450 mas), 1 sigma, radial. Following that, a target acquisition, either the MSATA or WATA method, can be used to place the science sources into their small science apertures with the accuracy required to perform subsequent science observations.
MSATA described below is the preferred target acquisition methodology 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.
Optionally, the WATA acquisition method can be performed with an offset star. A 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.
The following table describes the various TA methods, their expected performances, 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 Type||TA Approximate Accuracy||Description|
|Standard 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, are observed through the open micro-shutters. 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 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.|
|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 MOS planned without the use of the MSA Planning Tool (MPT). WATA is the default method to acquire most targets for high S/N spectrophotometric observations in bright object time-series (BOTS) mode, including transiting exoplanets in the brightness range 11.9 < J < 25.6 (Vega Mag) or, alternatively, 11.2 < AB < 24.9. 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. After centroiding the target in the wide aperture, an offset is applied to move the science target to the aperture that will be used for science. For BOTS, since the science aperture is the wide aperture, no final offset is needed.|
|VERIFY_ONLY||JWST pointing performance—0.45" (450 mas), 1 sigma, radial.||The NIRSpec VERIFY_ONLY method 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 or in a protected MSA configuration. This image can be used in post-analysis to assess the pointing accuracy. 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.|
|NONE||JWST Pointing Performance—0.45" (450 mas), 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.|
Near Infrared Spectrograph (NIRSpec)
NIRSpec Observing Guidance
JWST Proposing Tools of Interest to NIRSpec Users
Related JWST Observing Methods
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