An Actuator is provided for setting at least one optical property, in particular the position of a crosshair, of an aiming device, in particular of a telescopic sight, with a coupling part rotatable about an axis of rotation, onto which the at least one index element can be pushed. The actuator has a guiding device for guiding the index element onto the coupling part which makes it possible to push the index element onto the coupling part in the direction of the axis of rotation of the coupling part and to prevent a rotation of the index element relative to the coupling part when it is completely and/or partly pushed onto the coupling part at least in a direction perpendicular to the axis of rotation, and preferably in all directions.