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Differential flexure can occur when one part of the imaging system shifts slightly such as when a focuser (either on the guide scope or imaging scope) sags. The goal to beat this is to 1) use an Off Axis Guider or 2) make sure all parts of the guide scope (including camera) and imaging scope (with camera) is as rigid as possible and "moves" as a complete unit.
I added tube rings to the focuser and one on the autoguider to limit flexure at the focuser on the 80mm Short tube. So far so good, I was finally able to guide 6 min subs with the AT8RC (FL 1625mm) the other night.
Click here to see images of my current set up.