square as in parrellel to the image plane, is the tube mount bent on the corner, so that the cam is not mounted flat. Im grasping at straws here

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Dew can start creeping in from the corners on the cam. I just thought, do you have dew heaters on the scope OTA?. If you havent, get em youll have to eventually ayway, it could be dew, although it tends to grow evenly on the OTA front .
It appears to me there is vignetting in the corners too (elongated stars pointing towards the middle), I dont know tho, others may know if the QHY8 can vignet on the ED80, I wouldve thought it wouldnt have

, but it does look like it.