With clouds overhead I'm working on a motorized focuser project for the William Optics SpaceCat 51 APO refractor. For a prototype I'm going the Moonlite Focuser protocol, using João Brázio's Ardufocus (https://ardufocus.com) with an Arduino Nano microcontroller + A4988 stepper driver. For tests I'm using a NEMA 11 stepper with 1:100 ratio planetary gearbox. I am a fan of DeepSkyDad's products (I have an AF1) and I like his approach for using a custom 3D printed ring for the belt drive with his RedCat Autofocuser. I'm also a fan of K-Astec in Japan, and their approach uses a standard GT2 belt flipped inside out. Fantastic idea.
I have a batch of timing belts, and the best fit I have is a 2.5 mm Pitch (T2.5 Tooth Profile, 290 mm length, 10 mm width) you see in the pics. It's not perfect, and I'm waiting on the right sized pulley--so we'll see if this works. 3mm pitch might work better. Or I might have to go with DSD's approach and print a ring with matching timing belt pitch and teeth.
Some shots of the FocusCat project in progress: