[raymo]

Expansion and contraction are the cause of the focal changes, but the
effect is nowhere near as great as you describe, so something else
must have changed. Your focuser shouldn't have to be racked right in
to obtain focus. Have you got a Newtonian that has been modified to
make it suitable for imaging or something? I just wonder, because many Newts
don't have enough back focus for imaging. Skywatcher ones are fine, but many GSO
ones need to be modded.[ unless sold as astrographs].
raymo