The Twin Astrographic Catalog (TAC) Version 1.0

Abstract

A first version of the Twin Astrographic Catalog (TAC) of positions for 705,679 stars within $-18^{\circ} \le \delta \le 90^{\circ}$ has been produced. The sky coverage of the TAC is complete to over 90\% in that area. The limiting magnitude is about B=11.5. Positions are based on $4912$ plates taken with the U.S. Naval Observatory Twin Astrograph (blue, yellow lens) at epochs 1977--1986. The TAC is supplemented by proper motions which are obtained from a combination with a re--reduced Astrographic Catalog (AC). Some AC zones are available now and a complete northern hemisphere is expected by fall 1996. Proper motions of almost all TAC stars will be generated as the AC work progresses. The average precision of a catalog position is 90 mas per coordinate at epoch of observation. A large fraction of that error is introduced by the currently available reference stars. The inherent precision of the TAC data is considerably better. The precision of the proper motions is currently 2.5 to 4 mas/yr. Magnitude--dependent systematic errors have been found and preliminarily corrected. The final reduction of this plate material will be performed with the Hipparcos catalog in 1997. The TAC is about 3 times more precise than the PPM or ACRS in the northern hemisphere at current epochs and contains about 3 times more stars. The TAC has a higher star density than the Tycho catalog and provides independent, high precision positions for a large fraction of the Tycho stars at an epoch about 10 years earlier than the Tycho mean epoch. The TAC version 1.0 data are released as the AC zones become available. For latest information, look at the US Naval Observatory World Wide Web page this http URL