I asked Celestron themselves about a mirror lock on C8's and they said if you turn the knob counter-clockwise the mirror moves in the forward position and this is how the mirror should be moved when trying to final focus and avoid mirror movement during exposure.
With image shift it is usually a wear problem and can only be repaired, a crayford style focusser would help but you would have to make sure the crayford will lock also.
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