Quote:
Originally Posted by PRejto
Thanks for all the replies. Yes, the photo is the default stretch in CCDStack but magnified 400x. The only reason I'm looking at this is that stacks of luminance images taken with my KAF830 chip show a lot of these dark blotches. The individual subs also show the darkies but they seem to come to greater prominence after stacking and especially when stretching.
I've just gone back again and tried to see differences between uncalibrated and calibrated subs, and I've taken Rick's advice and looked at values before and after. I suspect something isn't going according to plan during calibration. The dark spots do not calibrate out.
Here are some typical numbers coming out of CCDStack (I selected the entire image);
Much appreciated if someone here can shed light on this data.
Thanks,
Peter
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Hi Peter.
did a quick back of the envelope and your flat stack should have a SNR of about 70, so what you have looks reasonable.
The splotchiness looks to me like normal stretched random noise (mainly shot noise). It will not calibrate out on an individual sub because a sub is a lot noisier than a stack of calibration images - the noise will change a bit, but it will only be at the stage of integration that you will notice improvements from calibration. If you do not get an improvement at stacking, then you have fixed pattern noise in the flats - most likely they contain imprinted dark noise. You cannot get rid of fixed pattern noise by integrating more lights - the only solution (apart from dither) is to improve the calibration by having more flats and darks - as you suggest.
Whole image measures of noise etc. can be very misleading because they incorporate hot pixels and image structure. The most reliable way to measure image noise is to select a small patch of background sky with no stars or obvious hot pixels.