Afilion01 Someone hypothesized that my ADU are too high because of gain. I will shoot between 5-8 ADU tonight and share the result.
I assume that your dew shield is extended to the identical position while taking lights and while taking flats, right?
If none of the fixes work, try placing the synthetic flat a little away (5 or 10cm) from the front of the dew shield, and cover the dew shield with some diffuse material (T-shirt held by rubber bands will do for now) to see if the problem becomes better or gets worse. That might help diagnose the problem. I.e., the miscalibration could be caused by from light from the synthetic flat reflecting off of the interior of the dew shields.
As another sanity check, you can take two sets of flats (together with two sets of dark flats). Use one set to derive the master flat, and use the other set (at much lower exposure to simulate a dark sky) as "light" frames. If the sensor is stable, you should see an evenly illuminated frame.
For what its worth, I use a Gerd Neumann panel, but on top of that, I also add a Daystar FlatCap diffuser to my objectives when I take flats. But I have never used an ASI294, and your problem may actually be specific to that camera model (unstable sensor) and not to the manner that you are taking the flats. If so, after finding the cause, you may be able to mitigate the problem, but not completely eliminate it.
Chen
