a joystick actuator for controlling the positioning of objects , for instance , displays on crts , having a pivotally mounted control stick operatively engaging and driving a pair of optical - mask members . the optical - mask members are retained for rectilinear sliding motion relative to fixed mounted opto - switches and include spaced fins positioned to interrupt light communication within respective opto - switches . positioning of the opto - switches and spacing of the mask fins are selected to provide successive unblocking of the interrupted light communication as the control column is displaced from neutral thereby encoding both the direction and magnitude of stick deflection .