The twin astrographic catalog on the Hipparcos system

The Twin Astrographic Catalog (TAC) is an astrometric, photographic catalog covering most of the sky between declinations +90 degrees and -18 degrees to an average limiting magnitude of about V = 11 and B=12. This release 2 i s based on the same 4912 plates as release 1, which were taken with the US Naval Observatory Twin Astrograph (blue and yellow lens) between 1977 and 1 986. The Hipparcos Catalogue has been used for a new plate-by-plate, rigoro us reduction. A significant improvement over the release 1 version of the d ata was achieved. With an average precision of 48 to 120 mas per coordinate , depending on magnitude, and a higher star density than the Tycho Catalogu e, the TAC is a significant catalog for proper-motion determination. This r elease 2 of the TAC contains high-quality positions of 705,099 stars, suppl emented by photographic photometry (B and V) for most of these stars. Astro metric reduction and catalog construction details are discussed and the fin al product is described, which is available from the US Naval Observatory.