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Historically, the accuracy of HST absolute astrometry has been limited primarily by uncertainties in the celestial coordinates of the guide stars as specified in the Guide Star Catalog. GSC 1.1 had nominal rms errors of ~0.5 arcsec per coordinate, with errors as large as ~1‐3 arcsec reported near the plate edges. This accuracy improved substantially in October 2005 (during Cycle 15) with the introduction of GSC 2.3.2, where rms errors per coordinate were reduced to ~0.3 arcsec over the whole sky. An updated version of the catalog (GSC 2.4.0) was released in _____MONTH/DATE______November 2017, 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 (SIs) 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 2.4.0. A summary of the GSC catalogs and associated errors over the HST lifetime is provided in Table 1.
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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 1.0 | 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 |
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” | 0.3” | |
GSC 2.3.3 | Oct 2009 | WFC3 installed May 2009 | ||||
GSC 2.4.0 | 2016 ??Oct 2017 | 2015.0 | 0.03” | 0.1” | GSC aligned to Gaia DR1 Citation? |
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