Since the start of JWST operations, several updates and improvements have been made to the Guide Star Catalog (GSC), FGS flight software, Guide Star Selection Subsystem (GSSS), and Operations Scripts Subsystems (OSS) as detailed in this article.

See also: JWST Pointing Performance, JWST Guiding Strategies and Challenging Fields

Improvements to the Guide Star Catalog

In December 2024, GSC3.1 became the operational catalog for the JWST Guide Star Selection Subsystem. The most substantial changes between GSC3.0 and GSC3.1 are summarized below.

• Correction of mismatched high proper motion GAIA objects to 2MASS objects
• Update Skymapper data from DR2 to DR4 (more accurate photometry & astrometry)
• Addition of NOIRlab Source catalog to provided deeper SDSS photometry
• Addition of VVV/VIRAC catalogs to provide additional sources and 2MASS photometry in part of galactic plane
• Revised star/non-star classification priority - now preferentially uses PS1, SDSS, Skymapper over GAIA
• Addition of a new field to the catalog (varRange) with a min(Mag)-max(Mag) value for identified variable stars

GSC3.0 was a major update from GSC2. GSC3 became the operational catalog for the JWST Guide Star Selection Subsystem in Spring 2024. The most substantial changes in the catalog are summarized below:

• For GSC2, the base catalog was based on Digitized Sky Survey photographic plates. GSC3 is based on Gaia DR3.
• Updates to GSC2 were based on photographic plate data augmented by measurements from modern CCD surveys. GSC3 updates are based on GAIA data augmented by measurements from other CCD surveys.
• The cross-matching procedure for GSC2 was based only on angular distance prioritization of matches. GSC3 uses a combination of angular distance and magnitude difference prioritization of matches.
• GSC2 was validated using statistics and SQL validation scripts, and IDL plots. GSC3 was validated via statistics, SQL, and Python validation scripts and plotting.
• GSC3 has improved astrometry, using recent epoch observations and improved precision.
• GSC3 has improved photometry due to the availability of more NIR observations and improved precision.
• GSC3 has improved object classification due to better input classifications from modern detectors and modern machine learning (ML) techniques.
• GSC3 has reduced artifacts by eliminating photographic defects around bright stars; additionally, object mismatches between early epoch plates and recent epoch CCD have been greatly reduced.
• GSC3 has incorporated a specialized catalog, the Galactic Nucleus Survey (GNS) into GSC3 to address the catalog errors and incompleteness in GSC2 around Sgr A*.
• Coverage gaps in the previous catalog have been filled, for example, in areas around very bright stars and in regions of very high extinction.

Improvements to the FGS flight software, Guide Star Selection Subsystem, and on-board commanding

Several tweaks to the FGS flight software have already improved guiding performance, and more changes are being investigated and tested. These improvements are listed below:

• Parameter updates to FGS flight software
• Monthly updates of bad pixel maps
• Limiting the area imaged to the central 1/3 of the detector for crowded fields. This improves the chances of the commanded guide stars and reference stars making it onto the "bright object list," which controls the targets considered for Identification.
• STScI are testing further possible changes to on-board commanding scripts and FGS FSW, especially for bright crowded fields (BCF).