AlexanderHornung Which motor do I have to order so that it really fits on the Vixen VSD 90ss?
The EAF can be made to fit, but the Q-Focuser is a much better focuser for a similar price point. Better electronic drives are available if you are willing to pay more.
You might be able to fit the high end Q-Focuser on the 10:1 step down side of the OTA focuser. That would practically get rid of backlash.
Which motor do I have to order so that it really fits on the Vixen VSD 90ss?
You can fit anything if you are willing to do some machining yourself. As I said earlier, direct drive your focuser. Remove that black knob to see what the shaft diameter is, and couple it to something like the EAF (which has a 5mm shaft). I would be surprised if that shaft has a non-common diameter, or even different from my FL55's shaft.
Chen