Re: [TASS] Flat fielding

["Arne A. Henden" <aah@NOFS.NAVY.MIL>]

Andrew,
  We discussed flatfielding to endless lengths about
a year ago.  The possible gradients in the flats is
well-known.  The solution that was decided upon was
to flatfield in the usual sense, and then apply a
correction factor based on photometry of Tycho stars
within the field.  Mike G. can discuss his ancillary
program that does this.  You have to flatfield if
for no other reason than to remove the vignetting.
In general, the flatfielding of a drift scan system
is far superior than for a stare-mode system since
everything is averaged over the entire column length
of the chip.
  Five percent standard deviation is acceptable,
considering that the internal error of any site is
on the order of three percent and you are combining
several sites.  This error reflects color transformation
errors, flatfielding errors, nightly zero point errors,
errors in the frame zero point from Tycho stars, errors
in the aperture magnitude calculations, etc.  These
are not great chips, the lenses have aberrations like
any cheap lens, the sky conditions are certainly not
photometric very often in the midwest.  Don't worry
too much about the scatter!
  What *does* worry me is that the mean magnitudes and
colors in tenxcat are not asymptotically approaching
the Landolt magnitudes.  This means that there are
still systematic differences between the sites that
could possibly be resolved if someone wanted to take
the time to study the problem.  If the standard deviation
of an 8th magnitude star is +/-0.05mag in the catalog,
then after 25 measures the average magnitude should be
within 0.01mag of the Landolt value.  This is not
currently the case.  Likewise, the finding that some
bright stars are being missed quite often needs to
be studied.  In other words, now that tenxcat is
available, we should consider it a 'first pass' at
creating a good catalog.  Study it, find the problems,
and then create it a second time.  You never get everything
right the first time.
Arne

--------------------------------------------------------------------
Arne Henden                               Instruments/software/CCDs
US Naval Observatory Flagstaff Station    Cepheids/photometry/IR
P.O. Box 1149                             ftp://ftp.nofs.navy.mil
Flagstaff, AZ 86002-1149                  Voice: (520)779-5132
aah@nofs.navy.mil                         FAX:   (520)774-3626