contains 526,280,881 sources, and is based on a
re-reduction of the Precision Measuring Machine (PMM) scans
from POSS-I O and E plates (>=-18°) and SRC-J and
ESO-R plates (<=-20°).
USNO-A2.0 was created by Dave Monet (Flagstaff Station, U.S. Naval Observatory.
The major difference between USNO-A2.0 and its previous version
USNO-A1.0 is that A1.0 used the
Guide Star Catalog as its reference frame whereas
A2.0 uses the ICRF as realized by the
USNO ACT catalog (Urban et al. 1997).
The VizieR search engine uses an
on-line compressed version
(3.6Gbytes) which was generated at CDS.