Format explanation of UCAC position only data ============================================= General notes: UCAC = USNO CCD Astrograph Catalog These are preliminary data obtained with the astrograph either as special observing or part of the survey. Positions are mean observed positions on the ICRF, linked by Tycho-2 reference stars. Various systematic errors (field distortions, magnitude dependent errors from CTE etc.) have been corrected similar to the UCAC2 reduction pipeline. Please refer to (and cite for publications): Zacharias,N., Urban,S.E., Zacharias,M.I., Wycoff,G.L., Hall,D.M., Monet,D.G., Rafferty,T.J. 2004, AJ 127, 3043 (May) "The Second US Naval Observatory CCD Astrograph Catalog (UCAC2)" Example table: RA SPD mag sigx sigy nobs rflag epoch mas mas 1/100 mas mas 1=ref 1/1000 411159390 559446221 1347 16 14 6 0 6930 07 36 50.6260 +65 24 06.221 412503213 559495172 1532 69 20 3 0 6932 07 38 20.2142 +65 24 55.172 411574302 559500791 1580 38 47 3 0 6931 07 37 18.2868 +65 25 00.791 ... Columns RA = right ascension (mas) SPD = south pole distance (mas) = declination + 90 deg mag = internal UCAC passband (579-643 nm) magnitudes, good to 0.1 to 0.3 mag sigx,y = internal, 1-sigma position error along x (RA*cosDec) and y (Dec) nobs = number of images (CCD frames) used for the mean position rflag = 0=field star, 1="good" reference star, else= ref.star excluded epoch = mean epoch of observation in 1/1000 year after Julian 1997.0, i.e. "6930" means 1997.0 + 6.930 = 2003.930 The position in HMS format is sometimes appended at the end of the line. Norbert Zacharias nz@usno.navy.mil U.S. Naval Observatory, Washington DC