In order to compute the position of a star on the sky from its image on a photographic plate, a mathematical transformation from the plate coordinates (x,y) to celestial coordinates (right ascension, declination) is necessary. The coefficients of this transformation are known as plate constants. The plate constants account for scale, plate rotation, origin offsets, differences in scale between the x and y axes, non-perpendicularity between the x and y axes, and plate tilt. Investigations are also made of systematic errors due to magnitude equation, radial distortion, coma, and measuring machines as well as those caused by measurers. When significant, corrections are applied. To determine the plate constants, the positions of several stars whose images are on the plate must be known a priori for the plate epoch. These are known as references stars; the catalog containing these data is known as the reference catalogue.
The reference catalogue used throughout the plate reductions of the AC 2000.1 was the Astrographic Catalogue Reference Stars (ACRS), available since 1991. This was compiled on the system of the Fifth Fundemental Catalogue (FK5), as realized by the International Reference Stars (IRS), and utilized the best data and reduction techniques available in the early 1990s.
Since its compilation, the FK5 has been superseded by the Hipparcos Catalogue. It was recognized that with the increased number of stars in the Hipparcos Catalogue over the FK5/IRS (118,000 vs. 40,000), the conversion of the catalogues making up the ACRS to the system of Hipparcos could be done more rigorously than the earlier conversions to the FK5 system. Since the systematic errors could be determined and removed more thoroughly, the final combined catalogue would be better. Additionally, the FK5/IRS data become very sparse fainter than magnitude 9.0, whereas Hipparcos contains over 35,000 stars listed as V=9.0 or fainter. This magnitude extension allows the removal of systematic errors in the fainter stars -- something that was problematic for the original ACRS.
In addition to Hipparcos, several new catalogues unavailable at the time of compiling the original ACRS have been released. The most notable are the Tycho-1 Catalogue, the Twin Astrograph Catalogue (TAC), FOKAT (Polozhentsev et al. 1989), and a recompiled Second Cape Photographic Catalogue (CPC2) .
Due to these improvements in astrometry, it was decided to compile a new reference catalogue. On this website, this new catalogue will be referred to as ACRS_1999. In total, the ACRS_1999 contains 391,838 stars which translates to an average of 38 stars per AC plate. The ACRS_1999 is not being released as a separate catalogue because the data used in computing the astrometry were subsequently used in computing the proper motions of the Tycho-2 catalogue. The ACRS_1999 can be thought of as a specialized subset of the data going into Tycho-2, which completely supersedes it.
Following the plate-by-plate reductions of each zone, the data were combined. The resulting catalog, called AC 2000.2, contains 4.61 million stars at the epochs of observation.
The introduction to the AC 2000.2 details the reduction method. It is available in PostScript and LaTex formats below or on the AC 2000.2 CD-ROM.
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.
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