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Hi,
when you say less than 1 arcsec resolution is needed, did you mean when measured at the motor end or the output shaft end cause I think the highest resolution optical encoders would be around 100,000 ppr encoders and that would be the resolution if they were to be mounted on the RA shaft.
I believe losmandy mounts use servos with integral motor encoders which I think is a closed loop system, so with all the high gear reduction, the resolution does work out to less than an arc second. but that wouldn't detect variation at the final output stage? precise machining of the ra worm and wheel might be key to its low backlash or PE errors.
I've geared up these encoders with a 3:1 timing pulley so its around 21000 ppr on the RA shaft at the moment.
60arc seconds might be sufficient for most goto uses.
also working on a closed loop system for steppers using feedback from these encoders output to XOR gates.
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