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And...... a typical imaging SCT is heavy at the back end and a typical refractor guide scope is heavy at the front end, so they droop differently, hence differential flexure (plus what Gama said).
This means that the guide star is kept in place in the guide scope with autoguiding, but the imaging OTA is outside the control loop so any movement between the guide scope and imaging OTA (flexure) shows in the image from the imaging OTA. Mounting stiffness between the guide and imaging OTA then becomes critical, this can be done in many ways, a never ending drama.
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