[Aavso-photometry] flat frame help needed

[Yenal Ogmen]

Hi all,
I've just started to experience flat fielding. I set up a white carton on the dome and project a white light on it. I am covering the telescope with a double t-shirt while exposing. I arrange exposure time in such a way to get readings between 20000 and 30000 ADU. I am taking 20 dark subtracted-flat frames then average them to have master flat. As you can see from the link below, flat frame looks good (isnt it?) but when I try to calibrate light frames with this master flat frame, I have this frame, linked below. Dark dust donuts that should disappear after calibration, turns out to be seen as white donuts. Dark vignetting turns into white vignetting. When I check the photometry, I see standard deviation of the stars increases. What am I doing wrong? Can anybody help me about this please?

 
Thank you in advance.

http://www.lookatcyprus.com/astronomy/yenalogmen/masterflat.jpg
http://www.lookatcyprus.com/astronomy/yenalogmen/calibrated.jpg


Yenal Ögmen
Green Island Observatory
www.greenislandobservatory.com
Geçitkale
North Cyprus