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Hi Mark, Some more info would be nice, like what sort of scope you are using.
However, I believe that this setup usually functions like this:
The alan key screw usually adjusts your tension on the focuser tube. The other is to lock the focuser so it can't move.
Usually you would adjust the tension with the alan key so that the tube doesn't slip when focusing near zenith, but isn't too tight that it makes the focus motion tight. Then you only use the thumb screw when you have achieved focus and don't want it to move. Generally you only need to adjust the tension every so often, once its set it will stay good for some time.
Hope this helps,
Dave.
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