Quote:
Originally Posted by pluto
The short answer is I don't think it matters, but I'm very open to being corrected on this.
My "lightbox' is made from an electroluminescent panel in a cardboard box. It's kind of blueish anyway and when I do my narrowband flats my Ha and SII exposures are about 5 times longer than my OIII images to get a fairly consistent white level.
Certainly when taking flats through LRGB filters the exposures will be very different lengths to get images with a count around 25k but I'm not using the flats to calibrate colour but just to eliminate gradients and dust etc.
However after seeing Ricks post I think I'll just chuck some sheets of paper over the panel and see how how much light they absorb.
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I don't think a bit of a colour cast would make much difference unless you had a scope with bad CA. As Hugh said it's common for flats with different filters to take substantially different exposure times. Even illumination is the important thing.
Cheers,
Rick.