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On 27 Nov 2004 22:17:10 -0500, (DoN. Nichols)
wrote:
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.
Nope. Focus ring is a nut, with a retainer to keep it from moving,
the male thread is on the lens barrel. Usually 4 to 6 lead though,
never single lead. Which means there are a lot of ways that it will
go together, only one of which is right.
Suppose a helical focuser for a scope could be made that way, but
most of them don't have a real thread as such, it's usually a radiused
groove, and engages a ball indent for rapid focusing. I make mine
like a very shallow acme thread, the groove just wide enough to take
the ball, and deep enough so the ball almost touches the bottom.
Parfocal eyepieces eliminate the need for the rapid, but add quite a
bit to the cost. There are focusers available in just about any
configuration anyone would want, if their wallet is heavy enough.
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