THE ACT REFERENCE CATALOG

The US Naval Observatory has completed the compilation of the ACT Refe rence Catalog, containing 988,758 stars covering the entire sky. The m otivation behind the ACT was to provide accurate proper motions for th e majority of the stars in the Tycho Catalogue. To do this, positions from new reductions of the Astrographic Catalogue (AC 2000) were combi ned with those of the Tycho Catalogue. The large epoch span between th e two catalogs yields proper motions about an order of magnitude more accurate than those found in the Tycho Catalogue. The astrometric data contained in the ACT Reference Catalog include positions and proper m otions and their corresponding errors. These are on the Hipparcos syst em (ICRS) for epoch J2000.0. Photometric and variability data from Tyc ho are included. In addition, crossreferences to the Tycho, AC 2000, B onner Durchmusterung (BD), Cordoba Durchmusterung (CD), Cape Durchmust erung (CPD), Henry Draper (HD), and Hipparcos catalogs are given. The ACT is now available at the international data centers and by contacti ng the authors via electronic mail.