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

Hello everyone, I plan to build an electric focuser using a NEMA 17 stepper motor from skysmotor.com.mx. 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 one 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!

Hello,
why do you choose NEMA17?
- what is it supposed to move and how? some kind of lens or set of mirrors?
- i've done "autofocuses" on micromotors, where the shaft is a worm gear on which a "kind of nut" moves under rotation; this moves the lens mounted in the mount moving on a "linear bearing slide" which sets the direction and maintains proper guidance.
- what does your lens moving gear look like?
- now the question, your focus is in what range of movement? if we are talking tens of mm, then there will be no problem, but if it is single mm or less, then you need to consider a suitable gearbox.
- is it supposed to be continuous autofocus or one-off?
- are you worried about vibration? how is the lens mounted in relation to the housing? i used to give 2 or 3 sliders in difficult cases, this makes the design a bit more difficult as they have to be precisely aligned, but eliminates any unwanted movement or vibration.
- if this is to work reliably in astrophysical conditions, then look for components designed for this, by using a Chinese motor, you are immediately condemning the project to potential problems. Is this supposed to work in orbit? Note that low temperature (very low) is a relatively minor problem with drastic conditions when exiting the atmosphere, presence of radiation, etc. Thermal expansion of metals or materials should also be considered for such operating conditions.
Regards