Quote:
Originally Posted by sil
Just make sure you loosen the mirror lock screws (don't go too far), get your focus, then slowly tighten them again (just watch through the eyepiece in case focus changes). As you move the scope to different parts of the sky the mirror can move (losing focus) so use the locking screws appropriately to hold the focus for the eyepiece.
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I think it's best not to use the locking screws at all for visual; focus needs to be adjusted frequently anyway, particularly if the scope is still cooling. If you approach final focus with an anti-clockwise turn of the focuser the mirror is much less likely to move.
An anti-clockwise turn pushes the mirror up the baffle. A clockwise turn pulls the mirror down the baffle tube, after which the mirror can settle/drop on it's own, slightly changing focus.