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I use a Lumicon OAG with a Meade DSI II Pro as the guide cam for a C9.25. I tried guiding on two other OTAs attached by Losmandy dovetails (which are fairly heavy duty) but couldn't avoid star trails.
So I definitely had differential flexure happening somewhere. Since using the OAG my shots have been fine - no field movement over several hours.
I recall guiding first using a Meade Colour DSI - gave poor results - the Mono version (well a higher model too) does a far superior job.
With the right tools, techniques and patience I may have been able to isolate and solve the flex problem - but I intuited it was the mirror shifting in the main OTA. Rather than invest all the effort - the OAG bypasses the problem for me.
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