Quote:
Originally Posted by Shiraz
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.
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Thanks very much for this Ray! I will be doing my flats over soon. I think the flat isn't working perfectly. It does remove the usual dust bunnies, but I'm noticing that the corners of subs are over corrected. I now wonder if I've got to get the light box closer to the objective because "something" is making the flat think it is darker in the corners than seems to be the case..
Peter