Geared stepper motor to main focuser shaft

This solution is most widely used in commercial focusers. It consists of relatively small stepper motor connected with proper gear box with ratio in the range 1:10 to 1:200. Shaft from the gearbox is connected to main focuser shaft. Backlash is present, as it is intrinsic quality of each mechanical gearbox, but it can be easily compensated with proper settings of focuser driver. This solution is also more expensive than other two, because there is additional cost of gearbox. 

Example stock solutions that are available for this solution:

• Nema 16 9V 0.6A motor with 1:14 gearbox
• Nema 17 12V 0.4A motor with 1:27 gearbox
• cheap 35BY412 motors with 1:42 gearbox available on amazon, ebay or aliexpress (I will test these motors soon)
• low power stepper motors available with gearbox at different retailers (like https://www.mclennan.co.uk/product/p535-series-geared-stepper-motor – can be pretty expensive)

Drawbacks of this solution are price and backlash. Advantages are: low power consumption, no power required when idle, compact size, high resolution and compatibility with many commercial solutions (Robofocus, Moonlite, USB Focus, Pegasus Astro, AstroLink and other). 

Custom ATM adaptation of stepper motor with planetary gearbox to main focuser shaft. Aluminium shaft coupler is visible in the picture between motor and focuser.

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Stepper motor to microfocuser shaft

This is quite easy solution that requires low power stepper motor, because torque to move microfocuser shaft is pretty low. All you need to do is to find suitable stepper motor for 9-12V. It can be both unipolar or bipolar motor, and it needs to provide torque at level 2-3 Ncm or more. Motor shaft needs to be connected to microfocusing shaft using a shaft coupler with proper diameter. 

Example motors that will work for this solution:

• Nema 8 geared Bipolar 1.8deg 0.3A 12V 3Ncm motor
• Nema 14 geared Bipolar 1.8deg 0.4A 12V 14Ncm motor
• Nema 14 geared Bipolar 1.8deg 0.4A 10V 5Ncm motor
• Round Nema 14 geared Bipolar 0.9deg 0.5A 8.5V 5Ncm motor
• Nema 16 geared Bipolar 1.8deg 0.4A 12V 21Ncm motor 
• Nema 17 geared  motor Bipolar 1.8deg 0.3A 12V 16Ncm

This kind of connections is quite simple and inexpensive. The drawbacks of this solutions are: microfocusing knob is not available anymore for manual focusing (however you can use shaft coupler to rotate it manually), and when microfocusing gear is not adjusted well, there can be some slip at its mechanism.

Stepper motor coupled with microfocusing shaft. No additional gearbox is required.