The Crayford design allows for fine, smooth adjustment in focus, and doesn't require the focuser tube to have a rack machined into it, so it's ideal for ATM's.
The helical Crayford looks like it would be a bit easier to print, so designed this one first. Spinning the focuser tube moves the eyepiece in and out, like a telephoto camera lens.
Nov 4 '11- Changed the design significantly. i'll probably put this up as a V2 eventually, but think of it as V1.5 for now. the bearings are attached to 'petals' similar to a camera iris, which are held closed with a large hose clamp.
I still need to add a focus tube lock screw, but check out the new design!
Parts list is preliminary.
Getting a properly sized aluminum tube will be tough. Does anyone know of a good, cheap supply of 1-1/4" (31.75 mm) ID Al tubing is used?
Anyway, I've got an idea for how to keep it tensioned on the tube, so stay tuned.
The mounting plate has more holes than necessary but the idea is that you glue the mounting plate to the optical tube, so i wanted to give extra mounting holes for other types of focuser designs. You'll need to put a m4 nut in each hole of the mounting plate before you glue it down, of course.
You should also glue the eyepiece lock into the focuser tube, but make sure an eyepiece will actually fit first. The holes on the lock are for M3 screws, and they are undersized so the screw self-taps.
The focus lock screw will probably be inconvenient without a knob or handle. it might also be good to have something on the other end of the screw in order to prevent the steel from marring the aluminum. something made out of PTFE would be ideal so that the lock would also be a clutch, adding some friction to the spinning.