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