Well, I think it's a collimation issue.
Setup is GSO RC10 at native FL, so around 2000 mm. No reducers or flatteners. Camera is ASI1600.
See the images below. I think my collimation is whacked, but I'm not sure what needs adjusting as it looked lined up in the tak scope. I've got a rotator on the system (nitecrawler), so if I rotate the image using it, the "defect" rotates as expected.
Any pointers appreciated.