The Smithsonian Astrophysical Observatory Star Catalogue (SAO) has been largely used as the reference catalogue in the reduction of astrometric plates in the past. These plates contain relevant information on comet and asteroid positions and motions.
The International Celestial Reference System (ICRS) is nowadays the conventional IAU Celestial System, materialized primarily by the International Celestial Reference Frame (ICRF). The optical realization of the ICRS is the Hipparcos Star Catalogue. The IAU recommended to perform all necessary ties between frames of different nature serving to various purposes, and the present realizations of the ICRS.
We compared the coordinate and proper motion differences between more than 100000 stars common to SAO and Hipparcos catalogues using vector spherical harmonic analysis. The results reveal distortions in the sphere defined through the SAO star positions, the most important effects being in declination, associated to a large displacement field towards the South pole. The coefficients of the series which represent the differences provide the tool to transform positions reduced with the SAO catalogue into the ICRS.
© European Southern Observatory (ESO) 2000
Online publication: July 13, 2000