The Third USNO CCD Astrograph Catalog (Entry Format) (UCAC3 at USNO)

The UCAC3 is a high density, highly accurate, astrometric catalog of 100,766,420 stars covering the entire sky. Proper motions and photometry are provided for all stars. Positions are on the ICRS (International Celestial Reference System) and given for the epoch J2000.0.

The UCAC3 is the third release of the ongoing UCAC project, designed to observe the entire sky over a magnitude range of 8 to 16, in the survey bandpass, which is between V and R. Observed positional errors are about 15-20 mas for the stars in the 10 to 14 magnitude range.

The UCAC3 has a number of major differences with respect to the UCAC2. These differences include:

• complete sky coverage
• re-reduction of the pixel data with better modeling
• double stars are resolved to the limit of the data
• significantly improved photometry from CCD data
• slightly deeper limiting magnitude with larger number of stars/area
• reduced systematic errors of CCD observations
• the addition of several new catalogs for improved proper motions
• photometry in the B, R, and I bands from the SuperCosmos project
• minor planet observations have been sorted out
• identification of more high proper motion stars
• match with 2MASS extended sources and LEDA galaxies

A UCAC4 release is anticipated in the future. The plan is to utilize a new reduction of the NPM plate material and benefit from global iterative solutions to further reduce possible systematic (zonal) errors and provide a strong link to the extragalactic reference frame. The UCAC4 release also might make individual positions at observational epochs public, and may serve pixel data as well.

Additional details will be published in the upcoming release paper (Zacharias et al. 2009) and in several technical papers describing details of the reduction procedures and results. For the latest updates see http://www.usno.navy.mil/usno/astrometry. The release paper should be cited whenever UCAC3 data is utilized.

Requests for the data DVD should be sent to the USNO. Technical questions can be addressed to Norbert Zacharias. The DVD will be sent to all addresses on the UCAC2 distribution list as soon as our resources allow.

Here is the data format from the distribution DVD. Here is the complete Readme file from the distribution DVD.

Software for searching the USNO UCAC3 has been added to WCSTools release 3.8.0.

scat (sucac3)	searches the catalog by sky location or number.
imcat (imucac3)	lists the UCAC3 stars in the region covered by a FITS or IRAF image with world coordinate system (WCS) information in its header.
imwcs (imwucac3)	fits a WCS to a FITS or IRAF image using the USNO UCAC3 as a reference.