the problem is not new but I still did not find a solution to this. I tried to search google but in every forums or user group I just find people with the same problem but no solution.
I am speaking of the problem that blue / OIII flats have a very strong pattern in them and are unusable, see the example image below:
The top row is (R, G, B) flats, the bottom row is (Ha, OIII) flats. The Ha and OIII flats have been taken with long exposure time (30s) but it doesn't matter, I tried very short (0,5s), medium (2s) or long exposure, all the same result.
This problem only occurs in blue spectrum so it has to be some kind of reflection. Is there a official statement from ZWO about this issue and how it can be solved?