Small(ish) update: We are investigating the issue that sometimes we do have a large reflection on the borders of the image. Good example is the Horsehead Nebula:
It seems to happen if there is a bright star closo to the border of the field of view. Moving the FOV a little can already solve the issue:
My recommendation is: Try to image a single frame in Luminance of your desired target and see if there are some glares. If not, you are good to go and continue. If yes, try to move the FOV by a little and test again.
For the time being this is the only workaround. We will take out the SII filter one of the next days and see if it is caused by the filters, plus look through the OTA with a searchlight and see if we can spot any reflecting border/ring/surface. Those fast optics are tricky...
I also plan on switching to Astrodon filters in the near future to reduce some halos (especially bad on O3 filter, due to a reflection from the corrector coatings), but we want to evaluate better the issues and their causes before doing big works. Once we move the camera the tip/tilt will change again, so expect possible disruptions during our testing.