How good is the astrometry for the images and the source lists?
HLA processing attempts to correct the astrometry of most images by matching sources with one of more of three catalogs, namely the Guide Star Catalog 2 (GSC2), 2MASS, and the Sloan Digital Sky Survey (SDSS) catalog; results are recorded in the astrometry keywords in the image header. The absolute astrometry is adjusted to match that of SDSS if a sufficient match is available, otherwise to GSC2; 2MASS is used to assess the quality of the solution. An analysis of the astrometric correction applied and its internal consistency for a subset of ACS data is presented in astrometry_all.summary. When a correction is possible, the typical absolute astrometric accuracy of the HLA-produced images is ~ 0.3 arcsec in each coordinate. About 80% of the ACS WFC images have corrected astrometry. This astrometric correction is not possible for many images because of crowding, lack of matching sources (especially for instruments with small field of view, such as ACS/HRC and NICMOS), or sources that are
