Elevation Axis For the elevation axis bearing, options considered  were flexures, flexural pivots, and conventional  ball bearings. Flexural pivots were chosen based on their high deflection capability, negligible  friction, and SDL’s extensive experience with  these devices in their Michelson interferometer.  Actuator choices for the elevation axis included  piezo actuators, magnetostrictive actuators, voice  coils, brushless DC motors, and stepper motors. A rotary voice coil was chosen primarily for its  favorable ratio of rotation angle to package size,  since only a limited rotation was required. Other  advantages to the rotary voice coil are simplicity,  low cost, and high bandwidth potential.  The moving coil of the elevation axis and the  mirror are bolted to a mirror mount, which is supported on flexural pivots. The pivots and the voice coil  magnets are mounted to a compact U-shaped yoke. The mounting scheme for the pivots is the same that is used in SDL’s Michelson interferometers. The pivots were sized to support the rotating mass without  failure during a representative launch load and to ensure virtually infinite life during on-orbit duty cycles.  Figure 3. Close-up of Wedge Sensor, Showing  Precision Wedge and Displacement Transducers The feedback sensor for the elevation axis was selected from the following options: non-contact proximity transducers (inductive or capacitive), strain gages, optical encoders, resolvers, and inductosyns.  Inductive non-contact sensors were cho sen for their low weight, compa ct packaging, low cost, high  resolution, and SDL’s related experience.  The feedback device for the elevation axis is unique and patent-pending. In this angular measurement  device (called the wedge sensor), two non-contact inductive displacement transducers are arranged in  opposition facing a moving target wed ge made of aluminum (see Figure 3). These transducers exhibit  excellent resolution and repeatability over a limited range. The precision wedge extends the range of the  non-contact transducers by converting angular motion to limited linear motion. Using two opposed  transducers minimizes the sensitivity of the device to relative displacements out of the plane of symmetry of the wedge (wobble, vibration, misalignments, etc). Nonlinearity in the wedge sensor over the large  deflection angles was mapped using a theodolite. This map was incorporated into the elevation axis  control algorithm.  Azimuth Axis The azimuth axis requires much larger deflections and needs to move a larger inertia. Consequently, the  bearing trades were more limited. Ball b earings were chosen due to their extensive heritage in space and  SDL’s experience base. Actuator options were brushless DC motors and stepper motors. Brushless DC  was chosen for its high bandwidth and accuracy capabilities and typically long service life. Feedback device options were encoders, resolvers, and inductosyns. An optical encoder was chosen for high  accuracy, low mass, and related experience.  The output shaft of the motor is connected to the yoke supporting the elevation axis. The optical encoder  has a resolution of 4.5 arcsec per count. Although this drive system is not designed for space, it uses the  same type of components as another SDL scan mirro r drive system, which has been operating flawlessly  for over six years in orbit. Upgrading the azimuth axis to space-worthiness has low risk.   Electronics Two independent sets of electronics are used to drive the azimuth and elevation motors. The azimuth  motor is driven using a commercial linear brushless servo amplifier. The elevation motor uses a simple amplifier circuit based on an operational amplifier.   389