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Greg, agreed.
The objective glass itself expands and in so doing the focal length shifts proportionately. For an objective used in a professional observatory this coefficient of change of focal length would be measured. It's desirable to have a tube with a coefficient of expansion that roughly matches that of the focal length, and aluminium is a better match than carbon fibre.
Some designs I've see went to the extent of including inconel spacing rods to tune the coefficient of expansion of the OTA to exactly match that of the objective.
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