[vsnet-chat 4644] Re: R: Question on V719 Aql+update position

[aah@nofs.navy.mil]

Toni wrote regarding the magnitude of V719 Aql.
USNO-A2 is an excellent astrometric catalog, but its
photometry is far from the best.  If you read Dave Monet's
introduction to the photometry for A1.0, he states
"The photometric calibration of USNO-A1.0 is about as poor as one can
 have and still claim that the magnitudes mean something."
USNO-A2.0 is better, but still has many regions of the sky
where the zero point is off by a magnitude and suffers greatly
at the faint end where insufficient calibration stars were
available.  In addition, it is a photographic catalog, which
means the photometric measures themselves have +/- 0.2mag errors.
Also, USNO-A2.0 is b and r magnitudes; any other magnitude
(say V) is a transformation from these two magnitudes and
has error as well.  Finally, this star is *very* bright for
the POSS plates and is just a saturated blob.  It is amazing
that the A2.0 catalog even gets close to the right answer.
  Finally, with stars this bright, you should be using the
Tycho2 catalog for your calibrations.  As an example, the
first star in your list:
USNOA2 195544.494 +072056.22 9.57/0.41
is given in Simbad as HD188613 with magnitude/color 10.35/0.34
which gives you a 0.8mag offset immediately.  Add in the
photographic blue -> Johnson V offset mentioned by Yamaoka-san,
and the star is about the right value.
  So rule of thumb:  use USNO-A2.0 for astrometry and a rough guess
at photometry, but use another calibration source if you want
the real photometric answer.
Arne

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