Starman asked:

Do all or most SCT's use a moving primary mirror for focusing?

Most of them do, although what is actually happening is that you are changing
the effective focal length of the instrument slightly and thus moving the
location of the focal point back and forth. This mirror movement can
introduce problems such as slight shifting of the image, which is one reason
many astrophotographers lock the mirrors and use an add-on external focuser.
Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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* Attend the 11th Annual NEBRASKA STAR PARTY *
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