The SuperCOSMOS Sky Survey - III. Astrometry

In this, the third in a series of three papers concerning the SuperCOSMOS Sky Survey, we describe the astrometric properties of the data base. We describe the algorithms employed in the derivation of the astrometric parameters of the data, and demonstrate their accuracies by comparison with external data sets using the first release of data, the South Galactic Cap survey. We show that the celestial coordinates, which are tied to the International Celestial Reference Frame via the Tycho-2 reference catalogue, are accurate to better than +/-0.2arcsec at J,R~19,18, rising to +/-0.3arcsec at J,R~22,21, with positional-dependent systematic effects from bright to faint magnitudes at the ~0.1-arcsec level. The proper motion measurements are shown to be accurate to typically +/-10masyr^-1 at J,R~19,18, rising to +/-50masyr^-1 at J,R~22,21, and are tied to zero using the extragalactic reference frame. We show that the zero-point errors in the proper motions are <=1masyr^-1 for R>17, and are no larger than ~10masyr^-1 for R<17masyr^-1.