I recently upgraded the focuser on my GSO 200mm f/4 Newtonian with a Moonlite low profile focuser. At a focal ratio of F/4 the critical focus range is 0.036 mm. I found I could not reliably focus by hand with the single speed focuser. I have used the Robofocus system for some time on my refractors so I thought I would look into adapting this system to the Moonlite focuser.
I have an adapter for the Robofocus motor to connect to the 8mm shaft on my Takahashi refractors. I just so happens the head of a #10-32 socket bolt is about 7.7mm. As luck would have it the diameter of the threads on a #10-32 bolt is approximately 4.8mm. This happens to be about the same size as the shaft on the Moonlite focuser! Finally if you get some water hose which is 3/16" (approximately 4.75mm) inside diameter here is what you do:
This was a nice case of having the right pieces around the house to rig up something that actually works.
I measured the step size to be 0.004mm and it takes 3600 steps/revolution of the focuser shaft. The backlash was less than 30 steps.
Here is a picture of the setup:
Who said astronomy had to be expensive?