Re: DS24 spatial variations

[Stupendous Man <richmond@a188-l009.rit.edu>]

  Andrew Bennett wrote:

> I have added a section on analyzing the spatial
> structure of DS24 images and flats to
> http://www3.ns.sympatico.ca/andrew.bennett/tass/mkivphot/mkivphot.html

  I agree with Andrew that the observed effects of clouds on 
stars are hard to understand :-/   Tom states that he can do a 
pretty good job of discarding cloudy images, and that is clearly
the right thing to do.

  In his document, Andrew suggests:

> In TN #90 Tom Droege points out that his flats suffer from north-south
> gradients due to sky light that vary with declination/zenith angle, in
> agreement with this investigation. He proposes to generate different flats for
> different declinations. I propose an opposite solution. I suggest it would be
> better to use one flat file and subsequently to apply a declination/zenith
> distance correction to the photometry.

  I agree with Andrew that we should use light-box flats.  However,
we differ slightly on the next step.  I would say

      a. subtract median darks (which are taken every night)
      b. use light box flats (which are taken frequently -- I don't
                know if nightly is best, but Andrew's idea of
                averaging several sets sounds good to me)
      c. apply a correction factor based on (x,y) position on the
                chip; this will be derived from grid tests

  Yes, the flattened images will still contain gradients due to real
changes in brightness in the sky (darker near zenith, brighter near
horizon).  However, this will NOT affect the photometry of sources,
since the Mark IV pipeline subtracts a local sky value from the counts
for each star.  There will be a very small difference in the limiting
magnitude of detected sources from North to South, but at an uninteresting
level; moreover, accurate photometry is much, much more important than
a uniform magnitude limit.

  If there were no scattered light problem, we wouldn't need step c.
If there _is_ scattered light (as seems likely), then we _hope_
it depends only on the position (x,y) in the focal plane.  There are
circumstances where that's not true -- for example, a neighbor's
porch light shines into the edge of the lens only when the camera
points as far east as possible -- but I shudder at trying to
deal with those.  Brrr.

                                      Michael Richmond