How to Choose the Right NEMA 17 Motor for an Electric Focuser?

[Beatrice]

Hello everyone, I plan to build an electric focuser using a NEMA 17 stepper motor. I have some questions during the selection process and would like to ask for your advice:

1. What is a suitable holding torque for a focuser? Is around 40 N·cm sufficient, or is a higher recommended?

2. Is it necessary to choose a NEMA 17 with a gearbox, or can microstepping drive meet the accuracy requirements?

3. Is there a significant difference in focusing accuracy between a step angle of 1.8° and 0.9°?

4. How can I reduce the impact of vibration and resonance on image quality when operating at low speeds?

5. Are there any special recommendations regarding motor current and heat dissipation in astrophotography environments (low temperature, long-term operation)?

I welcome experienced builders to share their model selections and troubleshooting experiences. Thank you very much!

[mens12]

Around 30–50 N·cm holding torque is typically sufficient for most focusers, and a standard NEMA 17 with microstepping (e.g., 1/16 or higher) usually provides enough resolution without needing a gearbox, though 0.9° motors offer slightly finer native precision than 1.8°.

To reduce vibration and resonance, use lower current, acceleration ramps, and a flexible coupler, and for long sessions set the current just high enough to avoid missed steps while lowering holding current to manage heat.