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Mirror lock:
You focus the primary mirror - then lock it, this prevents mirror travel around the threaded gears and hence de-focusing caused by "mirror flop" as you move your OTA around the sky. Then all you have to worry about is the tube length changing as the temperature falls and the body of your OTA contracts. You counteract that using electronic focusers, some of which have temperature sensors and automatic compensators for focus, and by always assuring focus by doing final focus against gravity to minimise flop.
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