2 Speed Focuser for Meade LX 200 10"

thingiverse

The focuser on my Meade LX200 10" telescope is a bit too coarse for me when using higher power eyepieces. I had planned to make one from a ball style reducer but couldn't find an inexpensive one suitable for the project. Then, I came across the "differential screw" by tkircher, which inspired this project. I wanted to retain the existing focuser holes in the telescope's optical tube assembly (OTA). This determined the maximum size of the housing and consequently the maximum size of the threads that could be used for the differential thread. As a result, I found the original focuser screw was too large in diameter to use in the design - so I made a new one out of brass with a slightly smaller diameter. This project requires PLA (ABS does not print the threads with an acceptable finish). However, I discovered that using PLA-CF gives a good enough finish for the threads to work. I ended up making the "Screw" and "SmallCyl" from brass and the "BigCyl" from aluminum in order to get a good "feel" to the focuser. Using short pieces of a coat hanger, I created the spline (2.04mm). Before machining, I drilled holes in blanks with a 2.5mm drill. Then, I glued the splines into the "screw" using loc-tite red. Design Flaw: The design is inherently flawed because the fine focus knob engages the differential threads and has more friction than the coarse focus knob that runs in the roller bearings. My solution (very ugly) was to pack the area between the housing and the BigCyl with some paper towel wrapped around the BigCyl and held with tape, which provides enough friction for the fine focus knob to move without turning the coarse knob at the same time. The 3D printed parts served as models for machining the parts. The collar is a very stiff press fit to the BigCyl - I recommend giving it more clearance. I will put the focuser in my Meade this weekend and let you know how it works. simplethreads.scad needs to be in the same directory as the focuser file.