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I have 2 OTAs, one Aluminium (12" SCT), the other CF (10" RC). The Aluminium tube requires focusing every time I use it and during a typical 6 hr imaging night. I usually dont bother refocusing during the night. The Aluminium tube SCT sub stars get noticabley blobier as the night wears on, especially if there is a large temp change.
Even though I always focus the CF RC on start up, I notice say with a typical 20000 count focus position, the new focus is never more than 10-30 counts off, and there is no visible change. The focus also does not change visibly during the night. Weeks can go by, with large temp changes, and the focus does not change visibly on reuse.
Unless there are other reasons for the SCT focus to change (the mirror is always hard locked, never changed), anecdotally at least, it seems to me, the CF tube is much better. Apart from other differences in these 2 OTAs, the focus in subs are always more consistant with the CF tube.
Having said that, with regular refocusing, the aluminium tube is fine, but unless you are fairly keen or have automated scheduled autofocus, regular manual refocus during long image runs can be pretty tedious, this is where the CF tube becomes very handy.
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