Greg I made a mistake! I had inadvertently ticked the optimise box for dark correction in PixInsight. This is a big no no when there is signal everywhere because you get under-correction for the dark. The PI algorithm fails under these circumstances.
Below you can see the first image is of 90 darks stretched to show all its glorious variation.
It is obvious where the waves are coming from! The rings are a bit like tree rings and they are due to variations in impurities of the original single crystal of Silicon the chip is made from. All chips are made from slices of a cylindrical single crystal of Silicon grown by slowly withdrawing a single 'seed' crystal slowly out of a melt of pure silicon. This is then usually zone purified to get even higher purities.
This is same NGC20270 data done with no optimisation of the darks. 10MB
https://www.flickr.com/photos/343863...in/dateposted/
My excuse I was tired and just wanted a quick result. My failure was to take more care and think it through. At 69 I suppose I am starting to lose it a bit.
Stonius I think that the noise is a tad higher at faster transmission rates due to the electronics converting and reading the data in the camera. I am sure that there is a Fourier type element involved.
The advantage of many darks especially at very long exposures is due to the amount of cosmic rays corrupting the data.
I also do a CCD flush before each exposure and for the bias and dark frames. You would be amazed what gets left behind in the wells from bright stars in exposures and cosmic rays in bias and darks.
I once thought I had found two comets in one field! It was just the leftover signal from two of the bright stars in Orion from a previous run made diffuse by dithering.
This does not matter normally but when one is pushing the system to get very dim stuff all the tiny artefacts start to look like some sort of signal.
We used to have a saying in CSIRO. "One man's noise is another mans signal." It is often difficult to separate signal from noise when one is groping in the dark.
Bert