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Just some history to PME/PMX etc.
The original PME's had 2.5 arc sec of PE. Problem was it took a day or two *just to set-up the worm* and measure the bearing run-out, to make that lofty spec happen.
So the next batch was 7 arc sec. The reason is purely one of repeatable tolerance in gears and the required 1/10,000th in the first production run simply wasn't possible without a very steep price increase.
Yet I find this all a bit moot. I have seen stars drift in Dec...i.e. *the axis the is not being driven at all* 2-3 arc seconds, so minuscule RA errors (even those afforded by absolute encoders/direct drive) become somewhat meaningless when the atmosphere is doing a Sterling-job meandering starlight across your CCD.
Sure, a solid and smoothly tracking mount is a good thing, but like Death & Taxes, I suspect guiding is the next thing you simply can't avoid...
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