Donuts after flat field

Hello,
I am having a problem with flats creating donuts out of my images after using the flats... Help!
What I mean is that before the flat the middle has a bright background and the edges are pretty dark, after the falt is applied the middle looks good (dark background) but the background towards the edge brightens up.
Thanks!
Chris Woodruff

Chris, we need details here. How are you taking your flats? How are you combining them? What is your optical system? What kind of counts are you getting in your combined master flat from edge to center? Can you post some examples of what you are experiencing?

dan kowall