THE TWIN-ASTROGRAPHIC-CATALOG (TAC) VERSION-1.0

A first version of the Twin Astrographic Catalog (TAG) of positions fo r 705,679 stars within -18 degrees less than or equal to delta less th an or equal to 90 degrees has been produced. The sky coverage of the T AC is complete to over 90% in that area. The limiting magnitude is abo ut B=12.0. Positions are based on 4912 plates taken with the U.S. Nava l Observatory Twin Astrograph (blue, yellow lens) at epochs 1977-1986. The TAC is supplemented by proper motions which are obtained from a c ombination with a re-reduced Astrographic Catalog (AC). Some AC zones are available now and a complete northern hemisphere is expected by fa ll 1996. Proper motions of almost all TAC stars will be generated as t he AC work progresses. The average precision of a catalog position is 90 mas per coordinate at epoch of observation. A large fraction of tha t error is introduced by the currently available reference stars. The inherent precision of the TAC data is considerably better. The precisi on of the proper motions is currently 2.5 to 4 mas/yr. Magnitude-depen dent systematic errors have been found and preliminarily corrected. Th e final reduction of this plate material will be performed with the Hi pparcos catalog in 1997. The TAC is about three times more precise tha n the PPM or ACRS in the northern hemisphere at current epochs and con tains about three times more stars. The TAC has a higher star density than the Tycho catalog and provides independent, high precision positi ons 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 relea sed as the AC zones become available. For latest information, look at the U.S. Naval Observatory World Wide Web page http://aries.usno.navy. mil/ad/tac.html. (C) 1996 American Astronomical Society.