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Text revised from HST Astrometry Project Overview
Improved WFC3 and ACS Data Products
Improved astrometry for WFC3 and ACS imaging data is now available from MAST, which includes two new corrections to the header world coordinate system. First, an updated Hubble Guide Star Catalog (GSC 2.4) was released which updated the coordinates of the guide stars with the positions from Gaia DR1. The typical uncertainties in the positions of the guide stars is reduced 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. When possible, an a posteriori correction has been applied to data by aligning sources in the images directly to the Gaia catalog. While some observing modes cannot be aligned to Gaia (e.g., grism and moving target observations), and the alignment can fail due to a lack of sources in either the HST data or the Gaia catalog, ~70% of the WFC3 and ACS data were aligned successfully. The typical uncertainty in the pointing is reduced to ~10 mas, although the uncertainties increase the farther in time from the Gaia epoch (2015.5).
Guide Star Catalogs
Historically, the accuracy of HST absolute astrometry has been limited primarily by uncertainties in the celestial coordinates of the guide stars. GSC1 had nominal rms errors per coordinate of ~0.5 arcsec, 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, which had rms errors per coordinate of ~0.3 arcsec over the whole sky.
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