[Robert_T]

Hi All, I've read a lot of posts on flats here. I went to do some this morning putting the DSLR into the scope pointed through a lighted window and stretched new white T-shirt material over the front of the scope and defocussed.

Now I've tried every exposure from 1/4 sec to 1/8000th sec and every single one has a narrow spike that goes off the scale ...now I know I'm meant to be aiming for 1/3rd pixel saturation. What's wierd is that the frequency of the spike shifts across the entire spectrum range as I change the exposure time. I can't work out what's going on. If I take the t-shirt off I get a normal histogram well spread out and not totally saturating the histogram. I'm stumped. HELP!