Depends on 1) your code (how many steps does it allow), 2) on the torque your stepper motor can deliver (if attached to the main axis, you will probably require microsteps to achieve precise focus resolution. Microstepping reduces the available torque and will likely result in missed steps, especially when the telescope is nearly vertical and has to lift the camera and everything attached to it, so I have my focuser (also controlled by Arduino) attached to the reducer axis to minimize the need for microstepping.

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