New Reductions of the Astrographic Catalogue: High Accuracy, Early Epoch Positions for Proper Motion Studies

Urban S.E.

U.S. Naval Observatory, Washington D.C., U.S.A.

The Astrographic Catalogue (AC) measures are being reduced at the U.S. Naval Observatory to provide early epoch positions for approximately 5 million stars. The data, when combined with recent observations, provide a baseline of about 100 years with which to compute proper motions. Due to the inhomogeneity of telescopes and measuring techniques used in the AC program, each of the 22 zones is being reduced independently, but with the same reference catalog, the Astrographic Catalog Reference Stars (ACRS). The data are analyzed and corrected for radial distortions, tilt, magnitude-dependent terms, coma, and distortions based on reseaux and measuring apparatus. To date, nine zones have been reduced and are available on the WWW or at the international data centers ( http://aries.usno.navy.mil/ad/ac.html ). Mean errors of the positional data vary from zone to zone, and are generally a function of measuring technique. Stars measured with the short screw method have errors in the 190-220 mas range per each image, whereas those measured with an eyepiece scale have errors in the 300-320 mas range. It is hoped that the reduction of all zones will be completed by the end of 1996.

An example of utilizing the Astrographic Catalogue data for proper motion determinations is being carried out at the USNO. The newly reduced AC data are being combined with the USNO's Twin Astrograph Catalog (TAC), whose plates were exposed during the late 1970s to mid-1980s. This program will provide a dense set of reference stars, with roughly 30% higher density than Tycho, in the northern hemisphere. This catalog should be available by the end of 1996. When the Tycho data are available, the AC positions will be used to improve the proper motions of the Tycho stars. In order to aid this work, the AC zones released by USNO have Tycho identifications, where applicable.