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Description
The Astrographic Catalogue (AC) was an international effort designed to photograph and measure the positions of all stars brighter than magnitude 11.0. In total, some 4.6 million stars were observed, many as faint as 13th magnitude. Users should realize that a few thousand of the brightest stars are not included; their images on the source plates were grossly over-exposed and therefore measurements were often not made. This project was started over 100 years ago, and the positions that have been derived from the AC data are being used, in combination with modern epoch positions, to determine accurate proper motions. The United States Naval Observatory has completed the reductions of the Astrographic Catalogue data (AC) to a consistent system. The resulting catalog, called AC 2000, contains 4,621,836 stars covering the entire sky, at an average epoch of 1907. The positions are on the Hipparcos reference frame (J2000.0) at the epochs of observation. Each of the 22 zones making up the Astrographic Catalogue was reduced independently using the Astrographic Catalog Reference Stars (ACRS). Each was analyzed for tilt, radial and tangential distortions, coma, magnitude equation and non-symmetric field distortions. Following these reductions, the data were placed on the Hipparcos system and the magnitudes were converted to be close to that of the Tycho B data. The resulting data were then combined into the final catalog. Detailed information on the reduction methodology and input data can be found in the file "intro.tex", or in the postscript documents provided provided in the "ps" subdirectory. The data contain the positions (eq. J2000.0) at mean epochs of observation, magnitude estimates and accuracy estimates for each star. Cross identifications with the Hipparcos Catalogue, Tycho Catalogue and the Astrographic Catalog Reference Stars are provided to facilitate future work with these stars. Interested parties are encouraged to visit the AC web site at http://aries.usno.navy.mil/ad/ac.html.
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