One way would be to mount the motor on a sliding rail and move it (parallel with the motor shaft) with a linear actuator (or servo or...)
A bevel gear would be mounted on the tip of the motor shaft (say, the left gear in the following image).
Moving the motor would engage and disengage that gear from another bevel gear, an idler (the middle one). When engaged, the motor would drive the idler, which in turn would drive a third bevel gear (the right one) that is attached to the telescope shaft.
