Time for an update!
The powder coating is all done and it looks great.
But then I hit a bit of a road block - I couldn't manage to get the collimation right with the laser collimator. I splashed out and got a quality 2 inch sight tube/Cheshire combo, auto collimator and new 'hotspot' center spot.
I was also suspecting astigmatism and mirror movement as well so I also removed the mirror clips and mounted the primary on 3 small pad of silicone
goop.
Bingo!
I did a quick test the other night with almost full moon - stars finally look good near the corners. Corners of this test image are magnified 2.5x. Taken unguided with a 500D prime focus, no flattener or coma corrector etc.
I'm not sure what is causing the vertical spike on Alpha Centauri - maybe the focuser is reaching down inside the OTA too much? I'll have to look into it.
Comments and ideas welcome.
James