A collaboration between STScI and ARI has produced a new astrometric reduction of the Guide Star Catalog (GSC, Laskeret al.1990). This new version, GSC 1.2, has dramatically reduced the systematic errors present in GSC 1.1. The positions in GSC 1.1 are affected by plate-based systematic distortions which are largest at the plate edges (1.0″, north; 1.2″, south) (Taffet al.1990a). These positions also suffer from systematic errors which are a function of magnitude and radial distance from the

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... te center (Morrisonet al.1996). This effect is small for radii under 2.7° from the plate center, then rapidly increases producing an average offset of the faint stars (15m) versus the reference stars (10m) of 0.9″ at the plate edges.