Abstract
The AC 2000 is a positional catalog of 4,621,836 stars covering the entire sky. The average epoch of position is 1907. The data are from the images measured and published as part of the Astrographic Catalogue (AC). The positions are on the system defined by the Hipparcos Catalogue, having originally been reduced plate-by-plate using the Astrographic Catalog Reference Stars. In addition to the astrometric data, magnitudes (on the Tycho B system) and cross-referencing information are included. These positions are valuable for the computation of proper motions, and were combined with the positions from the recently released Tycho Catalogue to improve proper motions of almost 1 million stars. The resulting catalog is known as the ACT Reference Catalog (the name ACT is from AC 2000 and Tycho). Both the AC 2000 and ACT are now available on CD-ROM and can be obtained by contacting the first author above. A World Wide Web site describing various aspects of the reductions is also maintained by the Astrometry Department, US Naval Observatory.
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