[vsnet-chat 1112] Re: USNO A1.0 comparisons

[aah@nofs.navy.mil]

  Kato-san gave an interesting comparison between USNO-A and a CCD frame
for the N Oph 98 field.  He found a good relationship, with a few bright
outliers.  I agree that these are most likely blends.  The USNO-A star
finding and photometry routines have problems in the Milky Way (though
I think they do better than any other plate measuring group).  The POSS
plates are basically black there, so setting scanning parameters is
very difficult (even if you could separate stars!).
  The photometric calibration is very crude, basically set by the GSC.
USNO-UJ is better calibrated in that it used photometric sequences on
each of its plates.  My previous statements were that, in small regions,
the USNO-A differential magnitudes are pretty good but the zero point can
wander by large amounts.  This is especially true if the field lies on a
plate edge.
  USNO-A is probably incomplete at V=19 or so because of the requirement
that an object be detected on both the O and the E plates, which had
different limiting magnitudes.  It will be deeper away from the plane because
of less crowding/confusion.
  It would be very interesting to do the same test in an uncrowded region.
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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