Last Updated Jan 2001

Copyright © 2001 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.

The GSC 2.2 astrometric reduction used two reference catalogs, the ACT and the Tycho 2.

The GSC II has adopted the ICRS as its reference system, and uses the ACT and the Tycho-2 Catalogue as its astrometric reference catalogs. Below we give a brief summary of the catalogs and how they were used in the calibration. For a more complete description of the catalogs please visit the links to these catalogs.

At optical wavelengths, the ICRS is realized by the Hipparcos Catalogue . However, on Schmidt plates the bright Hipparcos stars are heavily saturated on the deep Schmidt plates as are the bright end of the Tycho stars. However, Tycho's fainter end (V>8.5) have measurable images on the Schmidt plates, so each survey plate can be directly tied to the ICRS through these fainter Tycho stars.

Both the ACT and Tycho 2 catalogs are extensions of the orginal Tycho catalog from the Hipparcos mission. These two catalogs combinedearly epoch ground based observations with the Tycho data to improve the accuracy of the proper motions.

The ACT is an all sky catalog containing 988,758 stars. It is a combination of a new reduction of the Astrographic Catalogue (AC 2000, Urban et al 2000) and the Tycho Catalogue (ESA SP-1200). The large epoch span between the two catalogs yielded proper motions about an order of magnitude more accurate than those found in the Tycho Catalogue. The astrometric data contained in the ACT reference catalog include positions, proper motions and error estimates. These are on the Hipparcos System (J2000.0) for epoch J2000.0.

The positions and magnitudes of the Tycho-2 Catalogue were derived in a new reduction of the Tycho data from the ESA Hipparcos satellite, which led to better positions for the Tycho-1 stars, as well as extending the number of stars from 1 million to 2.5 million. The proper motions given for nearly all the Tycho-2 stars were obtained by a new analysis of 144 ground-based astrometric catalogs, including the AC 2000, bringing the positions in these catalogs firmly on the Hipparcos system. This global reference catalog is 99% complete to V=11.0 and 95% complete at V=11.5. Positional accuracies range from about 10 to 100 mas, depending on magnitude. Proper motion accuracies are from 1 to 3 mas.

60% of the plates used the ACT reference catalog to determine the plate coefficients and the remaining plates used the Tycho 2 reference catalog. In all cases the ACT was used to create the astrometric mask. The reason for the dual use of the reference catalog arose because we started the astrometric reduction of the plates in (1998), before the Tycho 2 catalog was not available (early 2000). Since a significant number of plates had been calibrated using an astrometric mask created from the ACT we decided to continue to use astrometric mask but switch to the Tycho 2 for the plate modeling. From a reference frame point of view both of these catalogs are on the ICRS system and any slight deviations are well below our measuring error.