The paper describes an optical Alignment Mirror Mechanism ( AMM ) , and discusses its control scheme . The mirror 's angular positioning accuracy requirement is + /- 0.2 arc - sec . This requires the mirror 's linear positioning actuators to have a positioning accuracy of + /- 109 nm to enable the mirror to meet the angular tip / tilt accuracy requirement . Demonstrated capabilities are + /- 35 nm linear positioning capability at the actuator , which translates into + /- 0.07 arc - sec angular mirror positioning accuracy .