I would have to agree with Graeme here as I doubt flexure would be a great issue @ F5 unless your focuser is flapping around in the breeze. "Perfect" polar alignment is very elusive and as long as the shift (read field rotation) is not too extreme within any given subframe it doesn't really matter. In your case it sounds like it would only be a problem if you were taking 2 hour exposures and if thats the case good luck to you
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Mark