As mentioned several times, most catadioptric scopes focus by moving the primary mirror forward and back. That works well, but, especially in scopes larger than 6-inches, there is always some play between the primary and the baffle tube it slides on; this causes the mirror to tilt slightly and images to move across the field. If the movement is small, not much more than the width of Jupiter, 45 arc seconds, focus shift is a minor annoyance and nothing more. If focus shift is enough to move an object off the chip of a CCD camera or out of the field of view of an eyepiece, however, it is a problem, and the scope's manufacturer or the dealer should be consulted.
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