The Astrographic Catalogue (AC) was part of the international Carte du Ciel program (see History and Zone Facts page on this site) designed to photograph and measure the positions of all stars brighter than magnitude 11.0. (Actually, the brightest stars are missing due to their images being grossly over-exposed and, therefore, not being measured). In total, over 4.6 million stars were observed, many as faint as 13th magnitude. This project was started in the late 1800s; most observations were made between 1895 and 1920. To observe the entire celestial sphere without burdening only a handful of institutions, the sky was divided among 20 observatories, by declination zones. Each observatory exposed and measured the plates of its zone, subsequently publishing the plate measures.
The U.S. Naval Observatory (USNO) in collaboration with other institutions completed data entry of the printed volumes of the AC in the mid 1990s. Since then, USNO completed the data reduction of each zone and converted the positions to the ICRS system as defined by Hipparcos. The first version of this catalogue, called AC 2000 (referred to on this website at AC 2000.1), was released in 1997. In 1999, the Copenhagen University Observatory and the U.S. Naval Observatory partnered to produce the Tycho-2 Catalogue. At that time, it was decided to re-reduce the AC data, which form the basis for most of the Tycho-2 proper motions. The second (and latest) version, called AC 2000.2, has been publically available since October, 2001.
There are two major differences between the AC 2000.1 and AC 2000.2. First, the reference catalogue was more closely tied to the Hipparcos system in the latter version, hence the systematic errors are smaller than those in the 1997 release. This will be especially noticeable in the fainter stars. Second, the photometry for about 2.4 million stars in the AC 2000.2 is taken directly from the Tycho-2 Catalogue. Only about 1 million stars had Tycho photometry in the AC 2000.1. Additionally, the format for the data has changed slightly between the two versions.
The positions derived from the published AC data have been used in combination with modern epoch positions to determine accurate proper motions. In addition to the aforementioned Tycho-2 Catalogue, the UCAC2 Catalogue also utilizes the AC data in this way.
Those needing additional information than what is on this site
can contact Sean Urban using seu AT usno.navy.mil.
This site was last updated November 16, 2006.
Disclaimer for external links