jc010 The 8 point pattern is always in the same rotational position, that is, squared with the sensor, even when I rotate the telescope.
Jim,
This is from ZWO's ASI294MC Pro page:
See if your four screws that are holding the sensor window are also shiny stainless steel. If so, blacken them with Sharpie pen, or replace them by black screws.
The fact that you also have a problem at 45º points to other potential problems, too. See if you can get a thick black (matte, not shiny) paper and cut a rectanular hole that is smaller than the octogonal shaped aperture (but larger than the black part of the sensor shown in the above picture). See if that helps.
If you have a filter in between the camera and the OTA, and the filter's anti-reflection coating is only on one glass surface, make the anti-reflection surface face the sensor side. Good filters are not directionally sensitive, but there are a lot of cheap filters out there where direction matters - you pretty much get what you pay for in the filter world.
Franco, your secondary problem appears to have a four way symmetry, so check the screws to see if they are shiny. The thin bright spikes are of course the diffraction pattern of your spider.
The problem could well be internal to the IMX294 sensor itself. Sony never intended for that sensor to be use for such a large dynamic range. (FWIW, I don't see this phenomenon on my IMX571 based cameras.)
Chen
