Virtually all conventional lightweight mirrors are made by optically grinding and polishing an already lightweighted blank. Mirrors made this way always risk print-through to the optical surface. In some cases, this can be removed by a zero-pressure process, such as ion-beam polishing, although these processes tend to be slow and costly. Lightweighting after optical polishing is not an option for conventional materials as their inhomogeneous qualities and internal stresses cause the lightweighting to distort the optical surface.
NASA Goddard Space Flight Center created a process that helps significantly reduce the risk, time, and costs associated with producing lightweight mirrors for demanding instrument applications. The method employs a solid disc of single-crystal silicon (SCS) and calls for most of the polishing to be completed before light-weighting. Due to the extraordinary homogeneity of SCS, the distortion caused by traditional lightweighting processes is significantly reduced.
Goddard’s technology is ideal for use in environments in which cryogenic operation or high heat dissipation is required. SCS lightweight mirrors typically weigh about one-fourth that of a solid quartz blank of the same size, making them useful for a variety of instruments where weight is a concern
The SCS technology provides a cost-effective solution for applications including space-based imaging systems, military reconnaissance, satellite and unmanned aerial vehicles (UAVs), and fast-scanning or -steering mirrors.
Each mirror is a monolithic structure consisting of a face sheet with a highly polished front optical surface. In the new process, the optical surface is formed in a solid SCS blank either by conventional grinding and polishing or by diamond turning. The blank is then lightweighted using Computer Numerical Control (CNC) grinding. For critical applications, post-lightweighting polishing can be performed to further improve the optical surface. Due to the very small amount of material removed during this step, it produces no quilting or print-through of the lightweight support structure. At several points during the process, the mirror is heated to near its melting point to remove small crystalline defects caused by the fabrication process.
Each resulting SCS mirror features a homogeneous composition free of internal stress. These parameters inhibit distortion when cooling the mirror to cryogenic temperatures. Under such conditions, the mirrors maintain their optical figure to 1/50 wave root mean square (RMS) or better. At room temperature, SCS has a thermal conductivity about the same as aluminum and a thermal coefficient of expansion about equal to Pyrex glass. So SCS mirrors are extremely resistant to thermal shock and ideal for applications where high heat dissipation is required.