Re: 2" Non-rotating focus tube?

Hi JD,

JD wrote: 
I'm part of a team that is building a solar physics instrument.  We
have two places in the instrument where we need to be able to adjust
the spacing between lenses, and the adjustment should not rotate the
lenses relative to each other, should have better precision than a
simple rail system, and we would like to take the set of lenses out of
the instrument as a set without changing the spacing.

I have found a couple of solutions of the type we need e.g. Helicoid
barrel.  Unfortunately the first set of lenses is 50mm in diameter. The
second is 30mm in diameter.  The diameter is too large to fit in the
barrel solutions I have found.  I have checked most of the usual
suspects and the only options that I have found do not have a large
enough clear aperture.  (Thor Labs offers a helicoid barrel, which is
almost perfect - almost.  It can only hold 1" (25.4mm) lenses.

We would rather not make a custom part on this.  Does anyone have some
ideas for an off-the-shelf or slightly modified off-the-shelf solution?

How much mass does the device need to support, and how much travel do you need?

Some Crayford-type focussers can take a few kg load, and there is a wide selection with 2" barrels, and a few larger sizes. Motorized positioning actuators with digital interfaces are available off-the-shelf. Crayford and reverse-Crayford are continuous-contact friction-drive analogues of rack & pinion, often with adjustable friction load. They usually allow positioning precision approaching that of helical devices but without rotation of optical elements and without the backlash and angular slop often found in rack & pinion. With the motorized versions, digital read-out and repeatable positioning are possible.

Check Jim's Mobile http://www.jimsmobile.com/ngf_data.htm for some 2" and 3" barrel examples.

Best Regards,
John.
.

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