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Theo,
With a Carbon fibre tube body the distance between the front corrector plate and rear mirror cell is effectively temperature invariant. Given expansion / contraction of a material is proportional to its length, and that this it the longest part of the OTA - a carbon fibre body addresses the crux of the problem.
The focusing rod holds the mirror - I presume some 10mm - 150mm from the rear of the tube so its only expansion / contraction of that (and the read end of what is hanging off the OTA) that can affect focus.
As I mentioned 4" vs 24" of material that can change size is alot smaller problem to deal with. Using CCD Inspector to check focus barely chnages over the weeks!
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