[The_bluester (Paul)]

You have some real detail coming out there, the 6" certainly does well. I keep thinking of trying my ASI2600 on the C925 though it would be heavily over sampled and would want binning either at the time of imaging or in processing.

I couldn't find your earlier posts about calibration frames without trawling back a fair way in a search, but I recall you mentioning not using darks with the 2600 (And you have not here) but I found a good reason to use them on the weekend. I set up on Friday night to test the new multi star guiding in PHD2 and grabbed a couple of subs on M45 through high cloud and with the moon rising. The effect I noticed was that if flats were applied and darks are not used, it over corrects the background in the corners as vignetting will have the "real" light levels falling off in the corners but the dark current is quite even over the field, so it gets multiplied in the corners, or anywhere there is a cause of light levels falling off, like dust bunnies.

I have seen the effect before but those times I did not twig as to what was causing it. More or less it says that if you plan to start using flats for calibration you will want to use darks. I would imagine the longer the exposure time of the lights the more noticeable it will be. The attached in order are a single 900 second sub stretched in APP, one with a flat applied and one with a master dark and a flat.