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Hi Peter,
Sorry I did not see your reply.
Are my flats 20,000 average or maximum you ask? Good question - I don't know. Its just the value CCDsoft puts out. Now that I think about it I think its likely to be the maximum.
As Marc has pointed out your flats are most likely too bright.
I have just seen with my CDK17 which really needs good flats as it vignettes quite a bit with my large chipped camera - that I have many times gotten crazy flat results where the flat makes the outer vignetted areas too bright. What worked for me is to make sure a bias was also subtracted (even if you subtracted flat darks from your flat). Usually though that was when I was using adaptive darks which are not ideal anyway. Adaptive darks is when you use say a 10minute dark on a 5 minute light and software adjusts it to suit your image.
I have also had it where a flat was too bright and it had that effect. I read a paper suggesting flats should be a lot brighter and are ok up to 80% of saturation so I am a bit unsure of the correctness of that statement. But in my experience that is what happened.
I'd have to read the paper again to see what was different about that approach. The concept though, was that modern CCDs sensitivity to light is very linear (unlike film) with length of exposure. In other words the CCD is the same sensitivity at 5 minutes as at 1 minute unlike film where sensitivity dropped the longer the exposure.
Greg.
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