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As of November 2019, an updated version of the catalog (GSC 2.4) has now been released, improving the celestial coordinates with the positions from GAIA DR1 and reducing errors to < 30mas over the entire sky. Thus, after including uncertainties in the positions of the science Instruments in the alignment of the focal plane to the Fine Guidance Sensors (FGS), the total error in HST absolute astrometry is ~1 arcsec for observations made with GSC 1.1, ~0.3 arcsec for those made with GSC 2.3.2, and ~0.1 arcsec when using the new GSC 2.4. A summary of the GSC properties and associated errors is given in Table 1.
Table 1: GSC Properties
Catalog | Date | Mean Epoch of catalog positions | Typical errors | Worst errors | Total Error (including SI to FGS alignment) | Comment |
|---|---|---|---|---|---|---|
GSC 1.0 | June 1989 | 1-2” | ||||
GSC 1.1 |
Aug 1992 | 1981.8 | 0.5” | ~1” | ~1” | First version published to the community Used by HST operations prior to Cycle 15 (WFPC2 installed Dec 1993) | |
GSC 2.0 | Jan 2000 | Science target fields only | ||||
GSC 2.2.0 | Jun 2001 | (ACS installed Mar 2002) | ||||
GSC 2.3.2 |
Oct 2005 | 1992.5 | 0.3” | 0.75” | 0.3” | ||
| GSC 2.3.3 | Oct 2009 | (WFC3 installed May 2009) | ||||
GSC 2.3.4 | ?? | Current version as of October 2019. Citation? | ||||
GSC 2.4 | Nov 2019 | 2015.0 | 0.03” | 0.1” | GSC 2.3.4 aligned to Gaia DR1 |
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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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