[Wavytone]

Agreed. With the exception of a Byers drive, virtually anything based on commercial gears will have massive backlash. Possible alternatives:

- a final reduction that has no backlash, ie a friction or tape drive;
- harmonic drive; or
- an arrangement where the drive is permanently loaded in one direction so reversing doesn't experience backlash (hard to arrange and not compact).

The trouble with friction drives is coming up with an arrangement that allows slewing and yet does track at the sidereal rate without slipping. A clutch is one way, but not the only way. To drive the shaft, use a miniature differential gearbox with two motors driving into it (the output being the sum of the rotation of the two input shafts). One input shaft would be the RA rate driven by a stepper, you could drive the other shaft either manually with a wheel, or with another motor with variable speed. You can find differential gearboxes, a Chinese one would be OK.

Regarding the friction drives, I wouldn't get too obsessed with the possibility of slip or inaccuracy of the reduction ratio because in tracking stars, atmospheric refraction means the apparent rate in hour angle is slight variable, depending on elevation.