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Beginning 3 December 2019, improved astrometry for WFC3 and ACS imaging data is available from MAST and includes two new corrections to the header world coordinate system (WCS). The first includes an updated 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 <~100 mas over the entire sky. Combining this new information with the knowledge of the instrument distortions, an a priori correction has been made for all WFC3 and ACS observations in order to lock all HST observations onto a common absolute reference frame. When possible, an additional correction has been applied by aligning sources in each HST image directly to the Gaia catalog; this fit is 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 70% of the WFC3 and ACS 80% of ACS/WFC and 50% of WFC3/IR frames have been directly aligned successfully. 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).
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Catalog | Release Date | Mean Epoch of catalog positions | Typical errors | Worst errors | Total Error (including SI to FGS alignment) | Comment | ||||||
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GSC | 12.4.0 | Jun 1989 | 1-2” | |||||||||
GSC 1.1 | Aug 1992 | 1981.8 | 0.5” | ~1” | ~1” | First version published for the user community Used by HST operations prior to Cycle 15 WFPC2 installed Dec 1993 | ||||||
+ Gaia Fit | Dec 2019 | 2015.5 | 0.01" | 0.01" | WCSNAME='FIT_[REL|IMG]_GAIADR[12]' | |||||||
GSC 2.4.0 | Oct 2017 | 2015.0 | 0.03” | 0.10” | GSC2.3.4 aligned to Gaia DR1 WCSNAME='IDC*-GSC240', 'IDC*-HSC30' | |||||||
GSC 2.3.3 | Oct 2009 | WFC3 installed May 2009 | GSC 2.0 | Jan 2000 | Science target fields only; GSC2 summary | GSC 2.2.0 | Jun 2001 | ACS installed Mar 2002|||||
GSC 2.3.2 | Oct 2005 | 1992.5 | 0.3” | 0.75” | 0~0.3” | |||||||
GSC 2. 3.3 | Oct 2009 | 2.0 | Jun 2001 | ACS installed Mar 2002 WFC3 installed May 2009 | ||||||||
GSC 2 | .4.0 | Oct 2017 | 2015.0 | 0.03” | 0.1” | 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 Used by HST operations prior to Cycle 15 WFPC2 installed Dec 1993 | ||||||
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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