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I see from your signature that you have an LX 90. The optical tube on these ‘scope is a Schmidt-Cassegrain which has a moving primary mirror and a fixed secondary mirror.
When you focus using the focus knob, the primary mirror slides up and down the central baffle tube and this is what contributes to the slight movement of the image when you focus. Provided the movement is only small, that is okay and there is nothing wrong.
Cheers
Dennis
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