The AC 2000.2 is a revised version of the 1997 release of the AC 2000 (Cat. ). It was decided that the availability of an improved reference catalogue and the inclusion of photometry from the Tycho-2 catalogue would be sufficient to warrant a complete re-reduction of the data and a new distribution of the catalogue. The AC 2000.2 catalog contains positions of 4,621,751 stars at the average epoch of plate exposures for each star (average 1907). Introduction: 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 overexposed and therefore measurements were often not made. This project was started in the late 1800's, 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 partnered with Copenhagen University Observatory in completing the reductions of the Astrographic Catalogue data (AC) to a consistent system. The resulting catalog, called AC 2000.2, contains 4,621,751 stars covering the entire sky at an average epoch of 1907, although the actual range in epoch is from 1891 to 1950. The AC 2000.2 is a revised version of the 1997 release of the AC 2000 (Cat. ). AC 2000.2 positions are on the Hipparcos Celestial Reference System (HCRS, coincident with J2000.0) at the epochs of observation. Photometry is from the Tycho-2 Catalogue (Hoeg et al. 2000, Cat. )) for the 2.5 million stars in common; the remainder are derived from image diameter measures on the photographic plates. The new reductions were performed because of the availability of the Hipparcos Catalogue (ESA 1997, Cat. ), which was not available during the initial plate reductions of the AC 2000. (A conversion to the Hipparcos system was performed as a last step in the AC 2000 reductions, however). In the original AC 2000, the reference catalogue used was the ACRS (Corbin & Urban 1991, Cat. ). To improve the astrometry in this second version, a new reference catalogue was compiled utilizing 140 observed astrometric catalogues including several, such as Tycho, which were unavailable during the original reductions. Each of the input catalogues was reduced independently to the Hipparcos system (HCRS) utilizing the Hipparcos data. The individual catalogues were then combined into the 1999 version of the Astrographic Catalog Reference Stars (ACRS_1999). The ACRS_1999 is not available for public release since the data were subsequently used in the Tycho-2 catalogue, hence it is obsolete. However, details of its compilation can be found in the Introduction on the CD-ROM. Each of the 22 zones making up the Astrographic Catalogue was reduced independently using the ACRS_1999. Each was analyzed for tilt, radial and tangential distortions, coma, magnitude equation, and non-symmetric field distortions. The resulting data were then combined into the final catalogue. Detailed information on the reduction methodology and input data can be found in the Introduction (files in the intro subdirectory) on the CD_ROM. The data contain the positions (HCRS, coincident with J2000.0) at mean epochs of observation, magnitudes in B and V from the Tycho-2 catalogue where available -- else from image diameters --, and accuracy estimates for each star. Cross identifications with the Hipparcos and Tycho-2 catalogues are provided to facilitate future work with these stars. Additionally, the numbering from the original AC 2000 has been maintained. It should be noted that there is not a strict one-to-one correspondence between the two versions; the main reason is that some images now known to be from high proper motion stars were not identified as such in the 1997 version. However the numbering remains consistent. In other words, the star numbered "1" in both versions refers to the same star. Interested parties are encouraged to visit the AC web site at http://ad.usno.navy.mil/ac/ac.html

Cone search capability for table I/275/ac2002 (Combined table from original tables)