Flight miniature Risley prism mechanism

More optical engineers are choosing to use Risley prism devices to accomplish alignment and steering of optical systems. A Risley prism device consists of a pair of rotating wedged optic elements that redirect rays of light by refraction. By rotating each wedge independently, the originating ray can be steered to a new angle or translated within a cone respective of the wedge angle and separation of the prism pair. The automated miniature Risley mechanism (MRM) was designed and tested for space flight where the physical envelope was significantly constrained, only very low power was available, and a unique power-off hold function was required. The MRM incorporates a prism pair with a 19 mm clear aperture and 0.75° wedge angle; it performs at 6 rpm (maximum speed) with a beam deflection accuracy of 25 μrad, and a power-off holding accuracy of 8 μrad within a 64 mm (optic axis) by 58 mm (height & width) envelope. This paper describes the driving requirements for the MRM and how the MRM assembly was successfully tested to verify its space flight performance requirements. Some design features included in the MRM assembly are: the radial titanium isothermal optical flexure mounts, a direct-drive zero-cog motor, a 37 Hz bandwidth closed-loop control system; a unique inductive position sensing system; and a fail-safe flexure-type brake assembly.