tn89

[Arne Henden <aah@nofs.navy.mil>]

Michael and Tom,
   The main problem with the technical note is the procedure for
flatfielding.  Here, the data frames were combined to create
a master sky flat, which was then applied to the data.  Master
sky flats will typically have an illumination pattern that is
dependent on local nearby light sources, and also will have a
gradient across the frame due to the limited sky coverage seen
by the Mark IV.  In addition, of course, is any scattered light
that is present in the system.
   Scattered light is a real problem with unbaffled systems and
causes insidious effects.  Tom's attempt to clear up any potential
scattered light problem by using flocked paper is certainly a valid
trial.
   However, I have a few other suggestions as to experiments.
First, try the *same* reduction procedure, but without any
flatfielding.  You should get a smooth vignetting pattern, but
a comparison of that pattern to the flatfielded frames should tell
you whether the flatfielding is working properly (or is being
contaminated).  Second, Tom (once upon a time) wrote that he had
built a lightbox for his system; you might try some of those
flatfields and ratio with the sky flats to see if there are
residual systematics.  Finally, point the MarkIV to some completely
different part of the sky (like -30 or +70 dec) and take another
set of data; ratio the sky flats from this second set and see if
there are residuals.  I'd really like to see someone build an
integrating sphere for the Mark IV (we talked about this also)
so that we can be really sure that the incident light is uniform
from at least one source.
   Flatfielding wide-field systems is the most difficult part of
the process, as I have mentioned before.  For the Mark III, we were
able to correct the flats after-the-fact since the cameras were
absolutely stationary; such techniques won't necessarily work here.
Arne