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I purchased an Orion Cheshire today and got to work this afternoon on my 6" newt to check the alignment between the focuser and secondary mirror. As you mentioned Alex the secondary was not aligned to the focuser. I backed off the 3 adjustment screws and loosened the main centre screw to provide some tilt and rotation but no matter how much I tried I could not centre the secondary over the primary ( I was a quarter circle out and could only see two of the primary mirror clamp shadows in my view)
After 2 hours of cursing and swearing I thought maybe have a look at the primary mirror. When I pulled it out I noticed the gap between the mirror base and the frame under one of the clamps was at least 2mm to 3mm wider than the other two. So I pulled the mirror off, gave it a clean and reinstalled it so all three clamp locations had equal gaps after tightening ( I just nipped it up firm but not to tight )
Re installed the primary mirror and tightened up then backed off the secondary screws and found that I could move the secondary mirror to line up with the focuser tube and primary mirror ( I thought what the hell , I've nailed it ) Tightened up the centre screw and tweaked the 3 adjustment screws so everything was concentric and aligned with the crosshairs , centre bulls eye on the primary mirror etc.. Then used my laser collimator to check alignment which only required a minor tweak on the secondary and primary
So to summarise , I believe it was mainly the primary that was causing my ovoid shaped refocused star pattern as the primary was adjusted by others a month ago with some cork spacers and when I removed the mirror the gaps were not even ( 2mm to 3mm difference) in which the error would be magnified further along the optical system
It's raining for the next week or so I will have to wait to test the scope again but here's hoping I've nailed the issue
Cheers and clear skies
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