The main reason for using the USNO catalog (either the smaller A2.0 or the B1.0) is the high level of accuracy in the position of the stars. This allows much more precise astrometry when trying to locate an unknown object. For example the Minor Planet Center desires astrometry accuracy into the sub-arc-second level. The original Guide Star catalog could not provide this level so until the plates were re-measured to produce the USNO series accuracy was a limiting factor. As for using the USNO catalogs, many plate solving programs (AIP4WIN, MaximDL, MPO Canopus, Astrometrica) can use these catalogs when performing astrometry. Once an asteroids precise coordinates are known one can go to the Minor Planet Center (http://scully.cfa.harvard.edu/~cgi/CheckMP ) to determine what the object is. The Minor Planet Center is a fun place to snoop around in. Jerry Foote ScopeCraft, Inc. 4175 E. Red Cliffs Dr. Kanab, UT 84741 435-899-1255 jfoote@scopecraft.com