Comparing Tycho-2 astrometry with UCAC1

The Tycho-2 Catalogue, released in February 2000, is based on the ESA Hippa rcos space mission data and various ground-based catalogs for proper motion s. An external comparison of the Tycho-2 astrometry is presented here using the first US Naval Observatory CCD Astrograph Catalog (UCAC1). The UCAC1 d ata were obtained from observations performed at Cerro Tololo Inter-America n Observatory (CTIO) between 1998 February and 1999 November, using the 206 mm-aperture five-element lens astrograph and a 4K x 4K CCD. Only small sys tematic differences in position between Tycho-2 and UCAC1, up to 15 mas, ar e found, mainly as a function of magnitude. The standard deviations of the distributions of the position differences are in the 35 to 140 mas range, d epending on magnitude. The observed scatter in the position differences is about 30% larger than that expected from the combined formal, internal erro rs, also depending on magnitude. The Tycho-2 Catalogue has the more precise positions for bright stars (V less than or equal to 10 mag, while the UCAC 1 positions are significantly better at the faint end (11 mag less than or equal to V less than or equal to 12.5 mag) of the magnitude range in common . UCAC1 data go much fainter (to R approximate to 16 mag) than Tycho-2 data ; however, complete sky coverage is not expected before mid-2003.