The first US Naval Observatory CCD Astrograph catalog

The USNO CCD Astrograph (UCA) started an astrometric survey in 1998 Februar y at Cerro Tololo, Chile. This first, preliminary catalog (UCAC1) includes data taken up to 1999 November with about 80% of the Southern Hemisphere co vered. Observing continues, and full sky coverage is expected by mid-2003 a fter moving the instrument to a Northern Hemisphere site in early 2001. The survey is performed in a single bandpass (579-642 nm), a twofold overlap p attern of fields, and with a long and a short exposure on each held. Stars in the magnitude range 10-14 have positional precisions of 120 mas. At the limiting magnitude of R approximate to 16 mag, the positional precision is 70 mas. The UCAC aims at a density (stars per square degree) larger than th at of the Guide Star Catalog (GSC) with a positional accuracy similar to Ty cho. The UCAC program is a major step toward a high-precision densification of the optical reference frame in the post-Hipparcos era, and the first st age, the UCAC1 contains over 27 million stars. Preliminary proper motions a re included, which were derived from Tycho-2, Hipparcos, and ground-based t ransit circle and photographic surveys for the bright stars (V less than or equal to 12.5 mag) and the USNO A2.0 for the fainter stars. The accuracy o f the proper motions varies widely, from 1 to over 15 mas yr(-1). The UCAC1 is available on CD-ROM from the US Naval Observatory.