[Aavso-photometry] flat frame help needed

[Tom Krajci]

From: Yenal Ogmen <yenalogmen at yahoo.com>
> I've just started to experience flat fielding. I set up a white carton on the dome and project a white light on it. 
Do you have any light leaks around the focuser, CCD nosepiece, filter 
wheel, etc.?
> I am covering the telescope with a double t-shirt while exposing. 
Does the scope have a light/dew shield on it?  Do you use it all the 
time?  Or do you take it off while making flats?  (Consistency is 
important here.)
> I arrange exposure time in such a way to get readings between 20000 and 30000 ADU. 
Have you tested your CCD for linearity?  What results did you get?  What 
CCD do you have?  Does it have anti blooming?  Is anti blooming turned off?
> I am taking 20 dark subtracted-flat frames then average them to have master flat. 
You may want to make a master flat, and then have a master dark 
subtracted from it, instead of dark subtracting every flat frame.
> As you can see from the link below, flat frame looks good (isnt it?) 
It's a jpg - much information is lost...not available for analysis.  
Feel free to zip up some sample images and email them to me    
<tom_krajci at tularosa.net>  Send:  one sample flat, one sample flat dark, 
one master flat (dark subtracted), one bias, one master bias, one dark, 
one master dark, and a couple data/star images.  Also, tell me how you 
make your master calibration frames (average? median? K-sigma? other?)
> but when I try to calibrate light frames with this master flat frame, I have this frame, linked below. 
I don't know how you are applying the flat field correction.  We can 
talk about that after I see some of your FITS files.

-- 
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Tom Krajci
Cloudcroft, New Mexico
http://picasaweb.google.com/tom.krajci

Center for Backyard Astrophysics (CBA)
http://cbastro.org/ CBA New Mexico

American Association of Variable Star
Observers (AAVSO): KTC http://www.aavso.org/
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