Hubble Guide Star Catalog – Astrographic Catalog/Tycho

The Hubble Guide Star Catalog – Astrographic Catalog/Tycho (GSC-ACT) is an astronomical catalog. It contains the twenty million stars of the Hubble GSC, version 1.1, recalibrated using the U.S. Naval Observatory's ACT (Astrographic Catalog/Tycho). By 2009 it was considered obsolete, superseded by such catalogues as GSC-2.2, B1.0, A2.0, and especially UCAC-3.

GSC-ACT was a re-calibration of the Guide Star Catalog version 1.1 for Hubble. The original GSC dated to 1989, and one of the reasons for an update was improvements to star catalogs that took place after that time. In this case the ACT catalog was used, which had for example more stars and whereas the GSC 1.1 had 3rd order fit for plate distortions, the ACT has a five-order available. Another improvement of the GSC 1.1 was the GSC 1.2.

GSC-ACT uses five classification codes:
 * 1       Star
 * 2       Galaxy
 * 3       Non-Star
 * 4        Artefact