Comparing Tycho–2 Astrometry with Ucac1

The Tycho–2 Catalogue, released in February 2000, is based on the ESA Hipparcos space mission data and various ground–based catalogs for proper motions. An external comparison of the Tycho–2 astrometry is presented here using the first U.S. Naval Observatory CCD As-trograph Catalog (UCAC1). The UCAC1 data were obtained from observations performed at CTIO between February 1998 and November 1999, using the 206 mm aperture 5–element lens astrograph and a 4k x 4k CCD. Only small systematic differences in position between Tycho–2 and UCAC1 up to 15 milliarcseconds (mas) are 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, depending on magnitude. The observed scatter in the position differences is about 30% larger than expected from the combined formal, internal errors, also depending on magnitude. The Tycho–2 Catalogue has the more precise positions for bright stars (V≤ 10 m) while the UCAC1 positions are significantly better at the faint end (11 m ≤ V ≤ 12.5 m) of the magnitude range in common. UCAC1 goes much fainter (to R≈ 16 m) than Tycho–2; however complete sky coverage is not expected before mid 2003.