Re: Processing Data

[<aah@nofs.navy.mil>]

Tom,
  Some comments on your procedure.
  Be sure the darks really have the same exposure time as the
data frames.  Note as the outside temperature changes, the
bias level will probably change, something I can't calibrate
until we get decent overscan pixels.
  As Mike said, each sky frame used to make the median should
be scaled by its mode or median.  I use mode.  I throw out
any frame which includes a very bright star (subjective, usually
if it bleeds a lot).  This manual operation, of looking at all
data frames to hand-pick the set for making a flat, is the
time consuming process of reducing a night of data.
  In general, it is better to dark subtract before median combining
the flat frames.  Hot pixels will survive otherwise, since each
sky frame may be scaled differently.
  Flatfields are *not* constant over time.  Ice crystals will
appear and disappear.  Dust gets deposited.  You may not get
a really good flat one night compared to the next.  Since we
have no uniform screen from which to take flats, it is better
to do it every night and therefore randomize any systematic
effects caused by twilight, moon, Milky Way, etc.
  Dark current should be relatively constant at a constant
temperature.  So if you are really regulating the TEC at, say,
+/- 0.1C, then I might trust darks from night to night.  However,
remember that they are low level, and to get decent signal/noise
so that they don't contribute much noise to the processing, you
need to median-combine lots, not just 6.  This hurts your faint end
the most.
Arne