Quote:
Originally Posted by Joshua Bunn
I read somewhere that if you divide one master flat by another master flat, the result should be a perfectly flat or something like that, whats this about? Is this what you mean by a "mostly featureless result" Rick. Is it to tell if your flats are good enough?
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Josh,
To make a flat you take an image of a uniform field of light. The resulting frame measures the variations in intensity caused by the whole telescope and camera system. It captures optical vignetting, shadows caused by dust, variations in optical elements, differences in efficiency in individual camera pixels, etc.
To perform flat calibration, a light frame is divided by a master flat (each pixel value in the light frame is divided by the corresponding pixel value in the master flat). The effect of this is to "undo" the variations in illumination we measured when we recorded the flat frames.
If you perform flat correction on a flat, then you should get back the original uniform field of light that was presented to the scope. That's the mostly featureless result I mentioned. Any features or patterns you see when you divide one master flat by another are caused by inconsistencies between them (possibly caused by different patterns of dust on the filters, non-uniform transmission of light by individual filters, etc.)
If a master flat taken with a luminance filter does a good job of correcting a master flat taken with a red filter then you don't need both. If it doesn't, then you will need to take flat fields and create separate masters for each of the filters.
This is also a good way to test a "uniform" light source like a light box: take a bunch of flats, rotate your light box by 90 degrees and then take another bunch. Make two master flats and divide one by the other. If you get a nicely corrected uniform result then you have a good light box.
Hope that helps...
Cheers,
Rick.