In article ,
wrote:

[ ... ]

Helical focuser is also an option, and I've made a couple of those
too, each has it's benefits and drawbacks. Personal preference there,
neither has a clear advantage over the other unless you're putting a
camera on the focuser, in which case the helical is out.

A double helical -- a tube threaded inside and outside -- one
left-hand thread and the other right hand thread, with an inner tube
connected to the camera and the outer tube connected to the telescope,
and some kind of keyway to prevent rotation of the inner tube relative
to the outer tube and you are fine. This is pretty much the way the
focusing ring on SLRs is done. (Some older lenses on folding cameras
had the front element on a single thread -- but that did not need to
move as far to focus through the desired range. Moving the entire lens
to focus takes more motion.

Good Luck,
DoN.

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