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January 2024: For more information on Hubble Advanced Products (HAP), please see the following resources.
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SUMMARY:
On December 3, 2019, the first set of improved astrometry data were released in MAST. The World Coordinate System (WCS) in the image header of all WFC3 and ACS datasets were updated and may include one or more corrections. The first makes use of a new version of the Hubble Guide Star Catalog (GSC version 2.4.0) which updates the coordinates of the guide stars with the positions from Gaia DR1. This reduces the typical uncertainties in the positions of the guide stars to ~200 mas over the entire sky. Combining this with knowledge of the instrument distortions, an a priori correction was made. When possible, an additional correction was applied by aligning sources in each HST image directly to the Gaia catalog, referred to as an a posteriori correction. While some observing modes cannot be aligned to Gaia (e.g. grism and moving target observations) or the alignment may fail due to a lack of sources in either the HST image or the Gaia catalog, approximately 80% of ACS/WFC and 50% of WFC3/IR frames have been directly aligned. For these data products, the typical pointing uncertainty is reduced to ~10 mas, although the uncertainties increase for observations further in time from the Gaia reference epoch (2015.0 for DR1, 2015.5 for DR2). The software used to produce these drizzled products is described on the Pipeline Astrometric Calibration page.
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Table 1: Key Guide Star Catalog releases and associated errors
Catalog | Release Date | Mean Epoch of catalog positions | Typical errors | Worst errors | Total Error (including SI to FGS alignment) | Comment |
|---|---|---|---|---|---|---|
| GSC 2.4.0 + Gaia Fit | Dec 2019 | 2015.5 | 0.01" | 0.01" | WCSNAME= 'IDC*_FIT_*_GAIADR*' | |
GSC 2.4.0 | Oct 2017 | 2015.0 | 0.03” | ~0.2” | GSC2.3.4 aligned to Gaia DR1 | |
| GSC 2.3.3 | Oct 2009 | WFC3 installed May 2009 | ||||
GSC 2.3.2 | Oct 2005 | 1992.5 | 0.3” | 0.75” | ~0.3” | |
GSC 2.2.0 | Jun 2001 | Public Release | ||||
GSC 2.0 | Jan 2000 | Science target fields only; GSC2 summary | ||||
GSC 1.1 | Aug 1992 | 1981.8 | 0.5” | ~1” | ~1” | First version published for the user community |
GSC 1.0 | Jun 1989 | 1-2” |
HST Astrometry Project
The coordinates populated in the FITS headers of HST observations retrieved from DADS (the HST Data Archiving and Distribution Service) were derived based on the guide star coordinates in use at the time of the observation. As the accuracy in these catalogs were refined over time, the pointing accuracy of HST has also improved. Table 1 lists the catalog in use at the time of installation of the three main imaging cameras (WFPC2, ACS, and WFC3) and the typical errors at each epoch.
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