I’ve been using nema17 stepper motors with planetary gearboxes for all my telescope builds. They can have quite high backlash so for telescope drives I have recommended the high precision types that have about 20arc minutes of backlash in practice. This is divided by the final drive ratio, usually about 25:1 so you end up with about 1 arc minute at the telescope mount axis. Acceptable but noticeable.
Recently harmonic drive gearboxes have become more available for stepper motors. I havent seen a combined stepper and harmonic gearbox, so they must currently be bought seperately and joined. Quite easy to do.
I bought a couple of 30:1 harmonic gearboxes and installed one on a nema17 steppr I had spare. The build quality is good and although they are a little large than the standard planetary type, its not enough to be a problem.
They are specified as having less than10 arcseconds backlash, and the one I am testing has effectively none!
I would say they are a little bit more noisey than planetary types, but at tracking speeds and slow slewing they are quite enough.
I dont have a torque meter, but a simple test suggests the harmonic gearbox is a bit less efficient, with perhaps 15% less torque. probably not a problem for most, but needs to be considered.
Photo below of harmonic (on left) compared with a standard planetary.
Update as of 28 Nov 2024: I incorporated two of these into my 18” ‘Dobsonian’, replacing the conventional nema17 planetary gearboxes (high precision models). They outperform the conventional gearboxes by along way. The lack of backlash leads to an extremely response drive - goto’s are more accurate and the fine joystick control is superb. Photos here and here.
