an actuator system having a controlled element configured for rotary movement about a first axis relative to a reference structure ; a linkage system between the element and reference structure ; a first actuator and a second actuator configured to power a first degree of freedom and an independent second degree of freedom of the linkage system , respectively ; the linkage system having a first link configured for rotary movement about a second axis not coincident to the first axis and second link configured for rotary movement about a third axis ; the linkage system configured such that a first angle of rotation may be driven independently of a second angle of rotation between said first link and said reference structure ; wherein one of the first or second actuators is configured and arranged to drive rotation of the element about the first axis when the other is operatively locked .