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Clive, I thought I'd replied earlier so if similar comments show up from me sorry. I've got a 10" Skywatcher Dob and my experience so far does not leave me thinking that the tracking is clean enough to do long exposure regardless of field rotation.
Others might have different experience but my impression is that even with good alignment the object wavers in the field of view. I've not done much testing yet to see if I can improve that behaviour.
Id suggest some thorough testing of how well the mount keeps an object centered over time before chasing field rotation if thats not already done.
Others might be able to suggest how long an exposure is required before field rotation is an issue but you should be able to get a feel for the stability of the tracking with a long exposure as is.
Bob
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