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For focussing, it depends on the thread pitch of the focusser. Usually within 0.001" is certainly "precise" focussing. The speed of the motor/ step size should be based on this calculation. The .25 to 1 sec arc limit is usually for fine tracking ( in RA) and achieved by mini stepping the motor ie 1/10 of a step as per Bartel's drivers. I'm putting together a 1.8 deg/step stepper with a 60:1 gearbox and a final 20:1 roller drive ie 1300:1 to give fast GOTO movement at 4 deg/ sec and tracking at 0.5 sec arc.
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