Thanks Martin for the solidarity. I think I'm onto something after reading this thread from
cloudy nights. Looking for a clear night soon to try the approach of rotating the camera & corrector and seeing if the astigmatism moves or stays fixed, indicating if it's the primary mirror or the corrector. My hunch is that it's the corrector. I've had lots of past success with f4 but i take your point, they're a little more finicky but nothing good collimating tools can't handle. Actually want to check out the SkyWatcher 150P f4 (or f3.45 with it's native corrector). 515mm @ f3.45.. yeah ok, I'm interested. Anyway, will update once the next test happens.