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 a long 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.
