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Assuming you have the "usual" 1rpm requirement to give sidereal rate.
This gives a rate of 15 arc sec/ sec time with an input of 1 rev (360 degrees/ 60 sec) = 6 degrees of screw rotation every sec. Giving 2.5 arc sec/ degree rotation.
For the standard camera lens normally used on the Barndoor mount, a resolution of 10 arc sec would be allowable. (Users to verify)
The stepper motors are usually (basic) 1.8 degree per pulse.
if attached directly to the screw you need 6/1.8 (3.33 pulses) per sec to maintain the tracking rate and each pulse would only move the mount 1.8 * 2.5 arc sec = 6 arc sec. Well below the 10 arc sec limit.
Hope this makes sense.
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