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I remember a post by Roland Christen about piggybacked guide scopes that may be relevant. He advised that the piggyack guide scope had to be connected to the imaging scope and not the rings.
I guess it eliminates some flex as the main tube flexes slightly perhaps so would the imaging scope.
So what about this:
1. Measure the distance between bottom and top of the 2 scopes and get it as exact as you can.
2. Add another bar across both of the scopes and attach the guide scope/guide camera there.
See if that helps.
With the Vixen R200S the question would also be whether the focuser is free of flex (Newts have heavy gear hanging off at 90 degrees to the tube and is asking for flex). And also whether there is any mirror shift.
I would have though mirror shift would be more erratic but that's just a guess. So its probably as discussed differential flex between the 2 scopes (how good are those Vixen mounting rings?) and non squareness.
Best guiding practice today is off axis guiders or high quality encoders on the RA axis. If you were really keen then high quality encoders may pay dividends as its not dependent on a flex prone guide scope.
Greg.
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