I was not satisfied with my Vixen 102mm Fluorite single speed focuser. Although this mechanism has an excellent teflon tension adjustment that provides fine tuning of the feel of the mechanism, it is severely limited by the rack and pinion single speed design. This fine fluorite lens has such a sharp snap-to focus that merely a tenth of a turn on the focus knob takes it from too far out to too far in. I found that for $300-$500 I can completely replace the focuser assembly, or for a little over $200 I can buy a new attachment from Vixen that provides dual speed on the existing mechanism. I may still go for one of these nice items, but being a tinkerer, I wondered if I could make my own at a much lower cost. Well, instead, for now, I bought a $13 differential used in hobby cars. I chose one that has steel internal parts. I opened up the differential and removed all but the ring gear, drive shaft gear with shaft, and bearings. Reassembled back into the casing, I then locked down the center hole of the ring gear onto the shaft of my focus mechanism using the screw that normally holds the stock knob in place. Add a single bracket to attach the gear mechanism housing to the underside of the focuser assembly (there's an extra threaded hole down there), and a simple knob to the drive shaft and voila - a 3:1, smooth drive reduction for less than $20 and an hour of time or so. I particularly like the change in angle provided by the differential that puts the fine focus knob right where I want it to be. Coarse focus is still achieved by the stock knob on the left.
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