SIMBAD references

The Twin Astrographic Catalog (TAC) is an astrometric, photographic catalog covering most of the sky between declinations +90° and -18° to an average limiting magnitude of about V=11 and B=12. This release 2 is 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 1986. The Hipparcos Catalogue has been used for a new plate-by-plate, rigorous reduction. A significant improvement over the release 1 version of the data was achieved. With an average precision of 48 to 120 mas per coordinate, depending on magnitude, and a higher star density than the Tycho Catalogue, the TAC is a significant catalog for proper-motion determination. This release 2 of the TAC contains high-quality positions of 705,099 stars, supplemented by photographic photometry (B and V) for most of these stars. Astrometric reduction and catalog construction details are discussed and the final product is described, which is available from the US Naval Observatory.