Focusing telescopes with moving mirrors

Because Schmidt-Cassegrains and Maksutov-Cassegrains usually focus by moving the main mirror (Figure 6.2, p. 65), you may observe a couple of annoying effects. One is lateral image shift - the image moves sideways a short distance as you focus. This problem generally diminishes if you run the focuser through its range a few times to redistribute the lubricants.

The other problem is that if you are moving the mirror backward, it may continue to subside for a few seconds after you let go of the focuser knob. More generally, you cannot "zero in" on perfect focus by turning the knob first one way and then the other. There's a "dead zone" and when you try to undo a movement, results are somewhat hard to control.

For best results, always turn the knob clockwise first, to overshoot the desired position, and then do your final focusing counterclockwise. That way, you are pushing the mirror away from you, working against gravity and taking up any slack in the system.

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