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I know you are limited to backfocus distance but ultimately the lack of an OAG or self guiding with an SBIG style chip on the filter wheel (perhaps you could make one yourself) means flexure is likely to come about.
I think ideally the guide scope if you have to use one should be connected to the OTA of the scope it is guiding not a dovetail that can move slightly independently of the OTA - hence the difference.
OAG eliminates all this. I wonder if OS can make a corrector to extend the backfocus somehow so all this can fit off the back. Or a QSI style camera with a built in OAG or one of the SBIG varieties.
Another alternative would be the SBIG STi and lens kit autoguider that is light, comes with rings and could be fixed to the OTA easily as it weighs nothing and should cope with a short focal length scope.
A guide scope off to one side on a different dovetail may be hard to pin down to no flex at all.
Greg.
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