[Aavso-photometry] flat frame help needed

[Michael Koppelman]

You are _dividing_ by the flat, right? Yo should also normalize it so  
the median is 1.0.

M.


On Dec 12, 2008, at 12:58 PM, Yenal Ogmen wrote:

> 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?