Quote:
Originally Posted by lebowski
I see very different opinions on the forums from 20000 to 50000 adu. I am very confused
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It's trial and error with different camera/telescope combinations. The theory and the maths say you need to have the brightest flat possible for your system. In practice it's different IMHO. I have tried them both. For me I had to find values that worked consistantly and it took a while. With my camera flats would vary between 9000ADU up to 20000ADU for the same camera depending on the filter, barlow/magnification and scope huge. To give you an idea of the variance I get very good flat fielding on my 5" newt at 9000ADU. On the C11 with the hyperstar I need to go to 20000ADU. For the C11 at prime focus I ned to go to 11000ADU. The ED80 at prime is 10000ADU. Those are the numbers that give me good flat fielding so I don't try to make sense of it all anymore. It works for me.
The only thing I'll agree with is that if you get the dreaded inverse vignetting after flat fielding (i.e. edges brighter than center of field) then it is not necessarily a flat duration problem. Dark noise will tend to go bright after flat fielding and it's mostly located on the edges of your field if you have siginificant vignetting. The cure in this case is to increase your SNR and flat fielding will work as advertised.