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I reiterate and agree totally with Fred. The 8" I have has not had to have the focus changed in 4 imaging sessions. I check of course on each occassion and mid way through the run check again. It just does not move. My Tak scope on the other hand has to be focused each session and has to have it changed during a session on some occassions. Coefficiency of linear expansion would suggest a metal tube will move if the temperature changes by as little as 5 degrees and it progressively moves more and more as the temperature changes. 20 degrees will have quite a large change in focus. Personally, I would be prepared to pay the extra.
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