Re: TASS Flat Fields

[aah@nofs.navy.mil]

Glenn G. described a technique for making flat fields.  It won't work,
for two reasons:
  (1) it doesn't account for high-frequency (pixel-to-pixel; well, for
        the mark III, column-to-column) variations.
  (2) the natural sky variations are an additive feature, not a flat
      field (multiplicative) feature.  Your technique gets rid of stars
      in making the flat field (good), but leaves in any real sky
      variations (bad).
  (3) the 2-D surface fit is overkill for the mark III anyway, since you
       should only need a 1-D (flatfield vector) fit.
The only way you can make good flats is to illuminate the camera system
with some sort of artifically even illumination.

>        The second image (from set #4) above produced an image that was
>*perfectly flat* (no variations) with indicates that this procedure works
>*very well* on the two images that I have tried this on so far.
   Compare the photometry of Tycho stars distributed across the frame.
I bet you will find errors reflecting the size of the sky-contaminated
background correction you now have in the flatfield.
Arne