A replacement for the factory installed focusing knob, which allows precise setup and indication of the focus position for a range of eyepieces, adapters, and photographic equipment. No more refocusing guesswork!
The original scope's construction modification is minimal and easily reversible. But most likely will void your warranty. Be especially careful while handling the mirrors during full disassembing of the OTA.
Inspect the attached Google SketchUp 1:1 model file for all of the details and measurements.
Your comments are welcome!
Step 1: Major Modification
You need access to a milling machine to do this major modification for the factory focusing shaft. It must be completely removed from the telescope beforehand. Pay as much attention as possible to the optical mirror during the removal procedure.
The width of the groove to be milled depends on the diameter of the set screw you have. The grove is about 1.5 - 2mm deep in order to include the top narrow part of the shaft. The set screw should easily slide in the groove, but without much of freedom to prevent back lag.
Step 2: Scrap Some Parts
- I've took off this nice (12 revolutions 100 positions each) knob from a salvaged resistors bank. It's actually inspired the design. I've drilled it's plastic top for the fixed shaft, because the height of the knob is not enough to contain the moving focusing shaft all the way it goes. Plus, it allows me to connect the flexible hand focuser in the future.
- The brass rod, which will serve as the new focusing shaft, was made of the variable resistor's shaft taken from the same resistors bank box. It's diameter is just perfect for this job. The especially nice fact is that the standard drill bit has the exact fit diameter (8/32in) too.
They are all of the same diameter for this size of variable resistors, though, just make sure it's long enough (about 40mm).
- I've cut it from the base flat. The size could be anything a bit longer than your scope's focusing shaft. But the longer, the better. It's easy to cut it later as necessary.
- then I've made a short thread on the factory end with hand tools to match some wide nut, I already had. The nut will be cut off of the hex side, polished on the other side and epoxy-glued to the shaft later, so the precision of the thread doesn't matter much. It's better to make it loose a bit, so you could precisely adjust the nut to the perpendicular position, while gluing or soldering it in place.
- next step is to drill the brass shaft along the main axis to make a good fit for the scope's focusing shaft. I had a smaller hole already in it, so it was easy to make it bigger.
- the last thing with the new shaft is to drill and tap the set-screw hole on the side of the brass shaft. It is better to assemble the focusing knob on the scope at this point and mark the lowest possible point for the set-screw beforehand.
- In order to do that, I had to enlarge the focusing shaft hole in the scope's back to fit the new shaft in it with some tension.
Step 3: Another Scope Drilling
To fix the knob in place I had to drill and tap another little hole in the plastic back of the scope for a small screw to match the groove on the back of the knob. After the initial assembling, I've also marked the knob's own set-screw position on the brass shaft and made a small groove for it on the shaft, to make sure it holds well and wouldn't slipper.
Step 4: Final Thoughts
I've put some white lithium grease inside the fixed shaft, on its outside bottom end, and around the bottom nut. The set-screws are treated with a thread stopper glue.
Everything works flawlessly!
The knob diameter is very comfortable. The dial wobbling is only 2 points. The dial lock (on the side of the knob) allows secure locking of the focus, so nothing would ruin it by an accident.
I've adjusted the scale so the value 1100 is near the end of the thread of the focusing shaft. So I'd never unscrew it completely.
Make sure that all of the parts moves without much of the force. Do not over tight the parts!
The dial on the picture shows 486.5