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Re: TASS Flat Fields
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- Subject: Re: TASS Flat Fields
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- Date: Wed, 20 Aug 97 16:39:36 -0700
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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
(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.