I am working on a project to fully automate my telescope, and one problem I had was adjusting the focus. After some discussion, the best course of action is probably to use a stepper motor with an encoder (which I am using for the alt/azimuth as well). But I wanted to try an idea I had to modify a high torque servo to see how well it would work in this role.
The problem I had was my servo could only rotate 180° while the focus required about 570° for the full extent. My initial idea was to get a larger pinion gear for the rack and pinion mechanism, which would make the focus only require 180°, but I couldn’t easily (or cheaply) find a replacement gear that would fit. So instead I decided to modify the servo for continuous rotation by removing the metal stopper on the gear. This would allow for 570° rotation (or more) but I would lose position control. So the solution here was to replace the servo’s potentiometer with a linear pot and attach it to the slide for position feedback. In the video below you can see it in action. It’s not pretty, I just hacked it together with zip ties to hold everything in place for now for a POC.