Doug,

I'm indeed not using the 10:1 fine focus. The reason is that those are friction-driven(*), and, like crayfords, are sensitive to slip. One can usually hold/fix the coarse knob on the opposite side and then turn the fine knob, and will be able to move it. I think I even read somewhere in a manual (maybe from the Pegasus? don't remember) to explicitly not use the fine gear for motor focusers.
And if the CFZ is around 50μ, a step width of 0.13μ sound like clear oversampling, isn't it?

(*) at least those that I know; they use an outer cup, an inner axis, and three steel balls as "gear". Don't know if there are others that have a real planetary gear for this.

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