Fine-steering mirror technology supports 10 nanoradian systems

Abstract

The profusion of lasers and related sensors has created new demands for line-of-sight stability in precision pointing devices. Meeting these needs for space-based devices on platforms that vibrate because of moving parts of satellites and structural bending modes presents a problem that needs a technically creative solution. Operational scenarios indude earth observation, surveillance, laser communication, and high-energy lasers. These applications often require the best optical quality available. The difficulty of this design issue is compounded by the friction-induced errors of beam-steering components that have bearings. There is a series of fine-steering mirrors now available that meets these needs in space, aircraft, and land vehicles, as well as stationary environments. These mirrors are based on reactionless concepts that cancel steering forces and torques at the point where they are generated. Also important is the use of highly linear actuators, sensors, and suspensions. Suspensions are made to avoid bending the mirror, which is designed to be stiff and light for achieving high bandwidth. Technological progress has been made in developing high-bandwidth fine-steering mirrors by solving critical design issues associated with wavefront quality, positioning accuracy, and quick and fast operation. These issues affect the design of the suspension, actuators, mirror, sensors, and reaction cancellation components. This paper explains some of the techniques we use to achieve 1/20 wave surface accuracy over a 7 in. mirror surface, positioning accuracy of 0.02% of the travel range, 5000 rad/s2 acceleration, and control bandwidths up to 4000 Hz. Although individual mirror designs cannot meet all of these performance parameters simultaneously, any of these can be met and all of these can be approached in various combinations. Tests on these mirrors, when operating in a tracking configuration with a low-noise optical sensor, show that they can achieve line-of-sight stability as low as 10 nrad.