[vsnet-chat 888] Re: Magnitudes, sequences, etc.

[aah@nofs.navy.mil (Arne A. Henden)]

  Kato-san has given some good advice on observing sequences.  There
are a few commercial software packages, of course, that do decent
photometric extractions.  IRAF/DAOPHOT etc. are available free, with
a steep learning curve.  I am releasing some of my automated software
so that those observers of the FASTT variables that need photometry
software have something to use, and that software might be of interest
to this group.  The port is to Windows95, and the package will be
ready either this week (if I can get to it before my many travel plans
come to pass).
  Your experience at Ouda reminds me of typical Midwestern U.S. observing,
but I don't remember extinction changing that quickly, especially if it
stays clear.  We don't see that here in Flagstaff.
  As you go to fainter sequences (and especially in Galactic plane
fields like for Novae), you will experience 'crowded' fields where
stars will often have neighbors.  The three typical ways of handling
such cases are: (1) remove any star from a sequence that has a neighbor
that might contaminate its magnitude determination; (2) use the smallest
possible aperture to remove as much of the neighbor effect as possible;
or (3) use psf-fitting to iteratively fit the flux from all neighbors.
All three approaches work, but I would use them in the order given, with
psf fitting being the most difficult approach to master.
  I took some data last night (and will again tonight) of Nova Sgr 1998
field, deep within the Milky Way.  Very, very difficult to do good
photometry under those conditions.  The frames are basically white with
stars.  Try easier sequences first!
Arne

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