Abstract: A scanning and/or tracking mechanism for use in airborne obstacle avoidance ptical radars operating in the infrared region. The radar beam is scanned by passing it through a pair of in-line rotating optical wedges which rotate around the boresight axis and which are transparent to the infrared radiation. The wedges are separately driven by servo systems including hollow shaft torque motors with the wedges mounted in the hollow shafts thereof. A high resolution and low backlash shaft position encoder is utilized comprising encoding discs directly driven by the optical wedges and including a novel decoding system.