Astrometric calibration is required for efficient operations and to determine the absolute coordinates of observed sources in the sky. It relies on determining the mutual positions of the instrument and guiding cameras in the focal plane, on measuring the distortions of the instruments, and on the use of an accurate guide star catalog.
Focal plane alignment
The target acquisition and placement processes (see details here) rely on knowing the locations of the various instruments relative to the guiders. This information is derived from calibration observations and carried in the Science Instrument Aperture Files (SIAF, Cox et al. 2009, Cox & Lallo 2017 not yet released). The SIAF files contain parameters that provide coordinates transforms to and from four different frames: (1) the detector frame, (2) the science frame, which is an offset, flipped, and/or trimmed version of the detector frame, (3) the distortion corrected "ideal" frame, and (4) the v2/v3 plane, a spherical coordinate system that relates all instruments to the observatory.
An accurate distortion solution is critical for several JWST observing modes, e.g. NIRSpec multi object spectroscopy. In addition, for most observations dithers will be required to improve the PSF sampling, remove detector effects, and mitigate the impact of cosmic rays. A reliable dithers execution calls for good knowledge of the distortion in both the fine guidance sensor and the science instrument. The requirement is that the uncertainty on the field distortion determination for any instrument and guider is smaller than 5 milli-arcsecond (mas) RMS per axis.
Astrometric calibration plan
The JWST astrometric calibration will be performed by observing a field in the Large Magellanic Cloud. The focal plane alignment will be determined by imaging many stars with accurately known positions in both the guiders and the science instruments. The distortion calibration will consist in fine-guiding off of a star in one of the guiders while taking images of the calibration field with the respective science instrument. A source finding algorithm will locate the sources in these images and the stars’ centroids will be measured. The astrometric comparison with the reference catalog yields the distortion solution.
The absolute astrometric accuracy of mapping pixels to the sky for JWST images reduced with the default JWST pipeline is limited by the absolute pointing accuracy at a level of approximately 0.5".
JWST astrometric calibration field
The JWST Astrometric Reference Field is a 5'×5' area in the LMC and located in the southern continuous-viewing zone. The field is characterized by a relatively homogeneous distribution of stars well-suited for JWST calibration, thus facilitating the derivation of the distortion solution in all JWST’s instruments (Anderson 2008; Anderson 2016). In 2006 this field was observed with ACS on HST, providing a catalog of more than 200,000 isolated stars with positional accuracies of about 1 mas.
Incorporation of Gaia astrometry
The absolute astrometric reference frame of JWST will be based on the all-sky catalogs produced by the ESA Gaia mission. The astrometric catalogs for focal plane alignment and distortion calibration will be anchored on the Gaia reference frame, and the same is planned for the guide star catalog, thus leveraging the Gaia precision for JWST. For instance, Sahlmann (2017) investigated the astrometric agreement of common sources between the JWST calibration field catalog as derived from the ACS observations and the first data release of the Gaia mission. The offsets, rotation, scale differences, skew, and higher-order geometric distortions between both catalogs were determined and, under the assumption that Gaia data are accurate and distortion-free, a correction to the JWST calibration field catalog was derived.