The Second USNO CCD Astrograph Catalog (Entry Format) (UCAC2 at USNO)

The UCAC2 is a high density, highly accurate, astrometric catalog of 48,366,996 stars covering the sky completely from -90 to +40 degrees in declintaion, and going up to +52 degrees in some areas. The northern limit is a function of right ascension. 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 UCAC2 is the second release of the ongoing UCAC project, designed to observe the entire sky from about R magnitude 7.5 to 16. The observed positional errors are about 20 mas for the stars in the 10 to 14 magnitude range, with about 70 mas at the limiting magnitude of R about 16.

The UCAC2 has a number of major differences with respect to UCAC1. These differences include:

• much larger sky coverage
• reduced systematic errors of CCD observations
• positions given at a standard epoch (J2000.0)
• the addition of several new catalogs for improved proper motions
• photometry in the J, H, and K_s bands copied from the 2MASS project
• data in binary format for direct access
• inclusion of software to aid users in quick access of the data

UCAC2 is the last intermediate data release before the final, all sky catalog will be constructed. Observations will end around mid 2004; the final catalog is expected out in 2005.

The UCAC project, observations and first data release are described in detail in the paper Zacharias et al. AJ 120, 2131 (2000). A version of that and other related papers are placed on the UCAC Web page (http://ad.usno.navy.mil/ucac/. The paper describing UCAC2 is in preparation for AJ (2003). The data are freely available; however, any publication utilizing these data should make proper reference to the project and papers.

The first UCAC2 data release is described in this press release.

Software for searching the USNO UCAC2 has been added to WCSTools release 3.4.0.

scat (sucac2)	searches the catalog by sky location or number.
imcat (imucac2)	lists the objects in the region covered by a FITS or IRAF image with world coordinate system (WCS) information in its header.
imwcs (imwucac2)	fits a WCS to a FITS or IRAF image using the USNO UCAC2 as a reference.