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D. G. Monet (USNO Flagstaff Station)
The second version of the USNO-A catalog is now available. The principle difference between it and V1.0 is the adoption of the ICRF as the astrometric frame of reference (the "Hipparcos reference frame" to the non-specialist) as realized by the ACT catalog (Urban et al. 1997). It incorporates new astrometric and photometric algorithms and reductions, an improved treatment of the systematic astrometric residuals from the various Schmidt telescopes, numerical refocusing of the Precision Measuring Machine, and the removal of magnitude equation from the astrometric residuals. The primary distribution mechanism is electronic (FTP to ftp.nofs.navy.mil), but CD-ROM copies are available for bandwidth challenged users. The new version is slightly larger than the previous one because the shift between southern and northern surveys has been moved from -33 to -17 degrees of declination. Although it is based on the same raw detection database as the earlier version, USNO-A2.0 should have smaller systematic errors because of the astrometric and bright photometric calibrations offered by the ACT catalog.
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