I've recently come across a very odd problem which was very difficult to diagnose. I'm still not sure I fully understand precisely what happens nor how to avoid it. I noticed the problem with my Sony A7S and if I'm right about the cause then it will potentially affect Nikon cameras as well. Concentric coloured banding:
I took 30 flats at ISO 100 and averaged them to create a (bias subtracted) master flat. Now, you would think that one of the original (bias subtracted) flats would be properly calibrated by this master flat. But dividing followed by stretching gives the following (displaying the green channel only):
Ignore the horizontal stripes on the left hand side - they are caused by the Sony A7S split sensor. It is the concentric rings that are of interest.
Looking at part of the histogram of that flat I see the following:
Certain values are under-represented (the histogram notches) and this fairly regular pattern continues throughout the histogram. Here's another plot where I highlight only those under-represented values - the histogram notches:
So each notch in the histogram of the file being calibrated causes a concentric ring in the calibrated file.
The obvious question you may be asking is why the master flat doesn't have the notches in the same place. The thing is, I took that set of flats against the dimming twilight sky. So each flat had a slightly different brightness. Looking at the histogram of each flat, each one has the same notches at ADU values at 522, 558, 597 etc. But in the image these values appear in different locations so once the flats have been averaged together, the notches are completely absent from the histogram of the master flat:
To clarify a further point, if I choose one of the other flats to calibrate then the concentric rings will have moved - they move to the location where the pixel values correspond to the fixed notches in the histogram.
The same thing happens when calibrating lights - a concentric ring will appear in the background where the pixel values correspond to the notches in the histogram.
So is this a problem for stacked images? It is more than likely that the background sky brightness changes slightly from exposure to exposure and therefore so does the position of the concentric rings. This should reduce their amplitude in the final stacked image because they then average out. But if the lighting for all your flats is identical then potentially the master flat itself could end up with concentric rings.
One final point. The Sony A7S has regular histogram notches in all three channels. Nikon cameras have regular histogram notches (or spikes) in the red and blue channels because of the way they are scaled. I don't know about the green channel. So it is likely that Nikons can show the same behaviour. To the best of my knowledge, Canon cameras don't suffer from the same histogram notches and histogram missing values.
When I started to investigate this issue I had assumed that it was the raw compression (using histogram combing) that was the root of the problem. This turned out not to be the case - instead it was the semi-regular notches in the histogram that was causing the problem. What, exactly, causes the histogram notches in the Sony A7S, I haven't a clue.
Edited by sharkmelley, 26 November 2016 - 09:34 PM.