I'd like to know who thought of using a helical design on telescopes. They're hard to focus and if you change eyepieces, it's like a major career to get in focus again.
It's not a rack-and-pinion focuser, but the Kineoptics helical Crayford focuser works very well, and has a low profile (which might be a limiting factor for your scope).
The original helical focusers on telescopes were of the fine threaded type and, indeed, refocusing when changing eyepieces or, simply, when switching observers with more or less myopia can indeed require many frustrating rotations of the draw tube. There is also the problem of slop in the threads when they are of the single thread type, especially for fast reflectors.
As howardcano correctly points out the helical focusers of the Crayford Style, like those sold by Kineoptics, are way superior to threaded types in ALL respects. One rotation of the draw tube will move the eyepieces up or down by up to 3/4 inches and there is no slop at all since there are no threads. The high quality bearings make accurate focusing a breeze.
If you are willing to give it a try and have access to a drill press (yours or a neighbors) you can even make your own Helical Crayford in one evening and for less than 15$: see this part of a thread where I show how to make a very simple one which will look like this:
This focuser is very easy to make if you normally use a Paracorr type coma corrector since no draw tube is required to hold the eyepieces (the coma corrector acts as it's own draw tube). I believe Schmidt Newtonians also have some residual coma and owners do use them with coma correctors. If this is the case the design I show in my thread would work as is. If not, a draw tube will have to be purchased or made.
If you want to go a bit more fancy you could also build the Dual Speed Helical Crayford I invented last year which is what the thread is all about. You would then have a very accurate fine focuser which also allows a rapid swap of eyepieces without having to rotate the draw tube many turns to refocus non parfocalized eyepieces.
Edited by Pierre Lemay, 06 August 2019 - 09:08 AM.
