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Improved Absolute Astrometry for WFC3 and ACS

Improved Absolute Astrometry for WFC3 and ACS imaging data was released in MAST in December 2019. Calibrated data now include an updated WCS (world coordinate system) in the image header which is used to produce the combined visit-level drizzled data products. In the majority of cases, alignment is performed by matching stars in each HST frame directly to the Gaia DR2 reference catalog reducing astrometric errors to ~10 mas (see ‘Improvements in HST Astrometry’ for more detail). For fields with insufficient overlap with Gaia, the WCS is defined by the Hubble Guide Star Catalog (GSC 2.4) or the Hubble Source catalog (HSC 3.0), which correct the coordinates of HST’s guide stars with their positions from Gaia DR1 with typical ~200 mas accuracy. In January 2020 the WFC3 team published a Newsletter article describing ‘New Astrometric Solutions in WFC3 Images’ and highlighted new tools for 'Using updated astrometry solutions' and evaluating the quality of the alignment. 


In mid-February 2020, an error was discovered in the astrometry database for a subset of observations acquired after October 2017, impacting archival queries performed between 3 December 2019 and 26 February 2020. The corrupted WCS solutions resulted in drizzled data products with unusually distorted footprints, and these were shifted to a position near RA=00:00:00, Dec=00:00:00 in the MAST Portal, as defined by the RA and Dec of the reference pixel (CRVAL1, CRVAL2 keywords). Because the error may have impacted the results of duplication checking for the upcoming cycle, an announcement to alert users was made in the Late Breaking News of the Cycle 28 Call for Proposals (due 6 March).  As of 26 February, a fix has been made to the database, and the affected observations have reprocessed. Users who downloaded drizzled data products during this period are advised to re-retrieve their data from MAST. If desired, users can also manually align observations to the Gaia frame, following the workflow described in the notebook ‘Aligning HST images to an absolute reference catalog'. 


Posted 4 March 2020
Last Updated: 06/02/2023


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