Posted By James C Chandler Jr
I really like some of the Borg components, and would likely use a small borg helical focuser along with either the straight-thru erecting prism or alternately the 90 degree erecting prism, depending on needs. That is the crux of the idea-- A small OTA easily changed around for various purposes, and able to use the nicer eyepieces for low-power viewing.
The slide-tube gross focusing idea seems attractive. Slide tube for gross focus and then fine-focus with helical focuser.
Configure the length so that it reaches infinity focus with the slider almost all the way in, so that if I wanted to watch a small critter 10 feet away from the deck, there would be generous out-focus travel.
I don't have good mechanical imagination, and have been trying to think up a slide-tube mechanism that is smooth but with minimal 'wiggle'. Also a design fairly easy to fabricate, since I don't have fantastic fabrication skills either. But it is good practice.
Here is a crude photo of a crude sketch. Hope it is even readable. I suspect this design would not work good enough. Just an initial idea so maybe someone can tell me a design that would actually work.
All parts are aluminum except two plastic rings and stainless thumbscrews.
An eyepiece, straight thru erecting prism, or 90 degree erecting prism would fit in the turned aluminum attachment on the back of the aluminum slide tube.
The 'stiffener ring' is just a ring of aluminum on the outside of the Main Tube to give the focus-lock thumbscrew sufficient purchase, and keep the back of the Main Tube round. Alternately, the Main Tube could be turned down out of fairly thick aluminum tube to leave some thickness on the back.
The two plastic rings are 'linear bearings'.
Plastic Split Ring (A) might be Nylon, Delrin, or Teflon. It is affixed to the Main Tube so the the slide tube moves thru the Split ring. The ring would be machined to size and then cut a thin slice out of the ring, so the Focus Lock thumbscrew can get real tight when desired.
The Plastic Ring (B) is affixed to the front-end of the slide tube, and moves with the slide tube. The intended purpose of the front Plastic Ring would be to make dern sure that the slide tube has minimal 'wiggle'.
I suspect this design would not work because of tolerance issues-- That in some cases it might be too tight, and if I loosen it up a little more than too tight, in some cases it would be too wiggly. But MAYBE it would work as-is.
How about some better-simpler ideas? Thanks!
James Chandler Jr.