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Paul
I had exactly your problem with my 72mm WO guidescope as I had almost perfectly round stars but gradual drift between exposures. Droves me nuts and so I went to an OAG and was able to do 20-30 min subs. Anyway I still liked using the WO scope and it's handy if I want to do a wide field shot so I looked carefully at the system that I was using.
Believe it or not it was a 2c fix.
Underneath my connection between the WO scope - extention tubes and DSI camera, I figured that this is where I was getting a small amount of flexture.
So to stop this I placed a small bolt underneath the DSI which screwed into the mounting plate on my 10" scope. I then screwed the bolt up so that it contacted the DSI and put a little pressure on it so that it would negate any sag in the system.
Fixed and I can now do 20 min subs at 2500 mm using the WO guidescope. So what I'm
suggesting is to firmly anchor the camera end of your guidescope setup - like JohnG did
to minimize the amount of flex in the system.
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