[vsnet-chat 735] Re: USNO-A

[aah@nofs.navy.mil]

  First, a caveat:  while I work 40 feet from Dave Monet, I am no longer
involved in the PMM project.  Anything I say should be considered my
own opinion, with the understanding that you should check with Dave
for the 'real story.'  I'll mention to him that the catalog team should
think about giving a nomenclature recommendation.  You already mentioned
that USNO-A and USNO-SA have different sequence numbers.  I personally
still strongly prefer a coordinate-based nomenclature so that someone
doesn't have to have a copy of the particular catalog (or the original
paper) in order to locate objects.
  I did the photometric calibration for the USNO-UJ (yet another catalog!),
and that catalog has J magnitudes that are good to <0.25mag.  The USNO-A
is an astrometric catalog, and as such the photometry was not given high
priority.  All photographic catalogs give poor photometry in any case,
since plates are inherently nonlinear and have definitely different
passbands than the standard BVRI (at least, the POSS OEJFN plates do).
Probably over a limited magnitude range, in a limited area of a plate,
and with a limited color range, you can create photometric sequences
where the internal error is smaller than 0.2mag, even on Schmidt plates.
How much smaller sure depends on a lot of parameters!
  I don't have any favorite transformations, so your transformation to
V is as good as any.  You should compare it to the GSC magnitudes since
they are supposedly V-like, or (as I'm sure you have already done) to PEP or
CCD sequences.  USNO-A r should be pretty close to Rc since they use the
same zero point and the central wavelength is about the same (though
the F plates have considerably narrower bandpass than Rc), and you may
be able to get by without any transformation equation at all.
  Bottom line: for visual observers, if 0.1-0.2mag accuracy is sufficient,
catalogs such as USNO-A or GSC-II or APS *might* be adequate, and in any case
homogenize and extend the existing chart sequences.  For PEP or CCD
observers, set up your own sequences and don't try to transform photographic
magnitudes into BVRI.  If you can wait a year or so, the TASS mark IV survey
will give very good magnitudes down to V=14 or so, and SDSS should be geared
up enough to give very good magnitudes from V=15 to V=20 north of the galactic
plane.  In other words, chart sequences may be able to benefit from some of
these other surveys.
Arne

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