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I find around 20-30,000 adu gives me good flats that correct vignetting without overcorrecting.
I take either dusk flats with a netural white cloth cover over the scope or I do the same but with the roof closed during the day and enough light gets in but not too much.
4 second exposures up to 14 work usually. Dusk flats are a little harder in that if you are doing 7 filters you have to move fast so that you don't have lots of different exposure lengths should you do flat darks. Its simpler if there are only one or two different exposure lengths.
Greg.
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