[AAVSO-DIS] RE: Photometry Software

[aah at nofs.navy.mil]

As Richard said, you can do very precise differential
photometry between a variable and one or more comparison
stars in the same field, even if you don't know the standard
magnitudes for those comparison stars.  After all, what most
software programs give you is the difference between the
measured instrumental magnitude for the variable (V) and the
comparison (C), the so-called (V-C) value.  This holds as long
as you know that the comparison(s) are non-variable themselves.
  In PEP measurements, you add a second star, the check (K) star,
to guard against variability of your comparison star.
For CCD work, you can continue to use the same procedures,
just that you measure V, C, and K on the same frame at the same time.
This is what Dan does.  There are three major differences
between PEP and CCD work under these conditions:
  (a) for PEP, the V,C,K measures are taken at different times,
      so more photometric conditions are required.
  (cb for PEP, you can ajust the integration time between objects
      and also select quite bright comparison stars, thereby improving
      the basic differential measure since noise in the comparison
      star goes away.  For CCD work, you have to use a single exposure
      time and are limited in your comparison star choice to those
      stars in your field of view.  Therefore, there is less dynamic
      range, and the chosen comparison star may even be fainter than
      the variable.
  (c) For CCD work, you have many stars that are measured simultaneously,
      not just V,C,K.  For this reason, you may want to use multiple
      comparison stars to form a master comparison 'star', thereby
      reducing the noise in its magnitude.
So in general, if the comparison star is well exposed,
you can get excellent results with just (V-C).  The reduction in noise
by using multiple comparison stars may be below your measurement ability.
I suggest that you start performing variable star photometry with this
simple (V-C) and (C-K) approach before worrying about the finer details.
  When transforming a differential measure into the standard system
by using transformation coefficients, multiple filters, and known standardized
magnitudes and colors for your comparison stars, then also using multiple
comparison stars is a minor addition to the complexity and offers
accuracy improvement since you average standarization errors as mentioned
by Michael and others.  That is the method I use to get into the millimag
accuracy range, but until you gain experience in doing photometry,
the complexity just gets you confused.
Arne