Re: [TASS] Flat fielding

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

Andrew wrote:
>As I understand it, "known" magnitudes are used to fix
>the initial scaling.
>The same set of "known" magnitudes are then used to
>take out the N-S gradients inherent in the method of
>flatfielding.
>This should ensure that the magnitudes that emerge from
>the output end of the processing pipe are well and truly
>nailed down.
  Not true, and that is why I asked the question.  You
have to understand the limitations at each step of the
process.  For that, I highly recommend the detailed
discussions that have occurred over the past two years.
Let me give you just two examples.
  The zero point adjustment done by Star uses a set of
'good' Tycho stars.  The Bt/Vt native magnitudes are
transformed into Johnson V and Cousins I with my equations.
There are inherent errors in this process, especially
with the I-band transformation.  A far better method would
be to use Landolt standards on photometric nights, but
these nights are few and far between in the midwest, and since
you cannot adjust exposure times with drift scan systems,
many of the Landolt stars are improperly exposed.
  The Flatcomp program, which follows the zero-pointing
process, tries to take out gradients in the image.  This
process is not perfect since it uses only a relatively small number
of stars, each of which has inherent error.  The result
is the removal of large-scale gradients, but with a
slope error in the few percent range.  Combine this with
the fact that not all cameras are pointed to the same
declination zone, so that stars track across different
areas of each lens.
  There are lots of these sources of error.  You should
understand the process before you start calling on
the statistician gods.  Herb has started generating
technical notes to explain the data flow, and Michael's
technical notes indicate some of the error sources.
Perhaps these are not complete enough, and someone
needs to write a TN explaining the error sources?  Any
takers?  (Lots of work to do for TASS!)
Arne