Progress - I think
OK, I think I'll leave you all n eace for a bit on this one
I've experimented with a range of settings. Sky Flat Assistant allows you to set the target ADU for your flat and then gives you an error margin. The default is 10% which I had been blindly using. Dropping it to 1% made the calibration a lot neater. Its still not perfect - I am sure there is some dark magic involved that I don't comrehend, but its a lot better.
Incidentally, I suspect that the key is not so much the target ADU setting as consistency between the images. I didn't see that much difference between 18,000, 19,000 and 20,000 ADU. But there was a big difference if the allowable error is 10% rather than 1% - even with a median combine.
Does that make sense?
Pete
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