[vsnet-chat 2137] Re: Supernovae estimates using USNO SA2.0

[aah@nofs.navy.mil]

Kato-san wrote a couple of weeks ago (yes, I'm a slow reader!):
>...I wonder what happens if the photometry software is
>instead forced to use sparsely populated bright stars and sometimes very
>faint (17-19 mag) stars.  Although the existence of such faint stars may
>not significantly affect the astrometric accuracy, I suspect photometry can
>be affected.
  I don't think it is a problem, for the following reasons:
  (1) most supernovae occur in external galaxies, which tend
      to 'avoid' the Milky Way.  Once away from the galactic
      plane, USNO-SA looks very much like USNO-A; very few
      stars are missed.
  (2) supernovae are usually relatively faint (say, 15th mag)
      and therefore the 17-19mag stars should be reasonably
      exposed if the supernova is exposed properly.
  (3) there is more error in proper background subtraction
      (e.g. galaxy contamination) than in zero point setting
      for most amateur measurements.
  (4) since there is roughly a 0.25mag photometric error for
      most of the PMM stars, the more stars you include in
      your solution, the better.  That usually means using
      many fainter stars to form an ensemble mean.  My software
      handles this situation properly, and I'm sure Kato-san's
      software does as well.  However, using something like
      CCDSoft with its single zero-point star photometry will
      give poorer results.
Arne

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