[vsnet-chat 3599] re NSV 12041 [UCAC1 data]

[aah@nofs.navy.mil]

Some misconceptions seem to be propogating, so
I'd like to step in.  This is not exclusively 'dumping'
on John Graves, but he was the most recent offender.  Sorry, John!
  First, I will reiterate that you should not be using
USNO-Axx numeric "IDs".  These *do not* exist in the
actual catalog and depend on whether you are indexing
into each zone with zero-relative or one-relative programs.
They change between versions, will not be the same
with USNO-A3.0 or USNO-B and just add to the identification
confusion.  Please only use coordinates!
  Second, UCAC is based on a Kodak 4kx4k CCD on a 0.2m telescope,
not a 1.0-m telescope.  Pixel density is not the same as for
2MASS.  They are both current-epoch, however.
  Third, UCAC also does not have numeric "IDs".  These
again are being supplied by some access program -- I strongly
recommend not promulgating these non-numbers!  UCAC1 is just
an interim release of the catalog (we have 6 updates here
at NOFS already, with another due this month), and misses many
stars that will eventually be incorporated into the catalog.
  Fourth, USNO-A is exclusively from POSS-I plates.  These
are epoch 1950; there are no 1987 plates in USNO-A.  POSS-I
plates *are* contemporaneous; the red and blue pairs were
taken back-to-back. You may be confusing USNO-A with the
PMM pixel server, which has digitized POSS-II as well as
many other recent surveys.  Since USNO-A, 2MASS and UCAC
are all ICRS/J2000, the one arcsec difference between
USNO-A and 2MASS/UCAC is most likely just 50-year proper motion.
Arne

--------------------------------------------------------------------
Arne Henden                               Instruments/software/CCDs
US Naval Observatory Flagstaff Station    Cepheids/photometry/IR
P.O. Box 1149                             http://ftp.nofs.navy.mil  
Flagstaff, AZ 86002-1149                  Voice: (520)779-5132
aah@nofs.navy.mil                         FAX:   (520)774-3626

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