The Guide Star Catalog, Version 1.2: An Astrometric Recalibration and Other Refinements

ABSTRACTA collaboration between the Space Telescope Science Institute (STScI) and the AstronomischesRechen-Institut has produced an astrometric re-reduction of the Guide Star Catalog (GSC) published byLasker et al. This version, GSC 1.2, has dramatically reduced the systematic errors present in the GSC1.1. As shown by Taƒ et al., the positions in the GSC 1.1 are aƒected by plate-based systematic distor-tions that are largest at the plate edges in the north and in the south). In addition, Morrison et(1A.0 1A.2al. have shown that these positions also suƒer from systematic errors which are a function of magnitudeand radial distance from the plate center. This eƒect is small for radii under from the plate center,2i.7then rapidly increases, producing an average oƒset of the faint stars (15 mag) versus the reference stars(10 mag) of at the plate edges. Since the magnitude term was similar for the two main plate collec-0A.9tions comprising the GSC, which were taken with diƒerent telescopes, exposure times, emulsions, and—lters, we believe this eƒect is common to fast wide-—eld-of-view Schmidt plates. In this paper we reporton the astrometric quality of the GSC 1.2 and present the method we developed (for large platecollections) to quantify and reduce the positional errors which are dependent on location on the plateand magnitude. Instruction for obtaining the catalog may be obtained from the STScI World Wide Website, or the user can obtain the catalog directly from CDS (Centre de astronomiques deDonnee“sStrasbourg). This version of the catalog has not been installed in the HST Guide Star Selection system.Therefore this version must not be used for HST observation planning.Key words: astrometry ¨ catalogs ¨ methods: data analysis ¨ surveys