High Quality Optically Polished Aluminum Mirror (GSC-14147)
Researchers at NASA Goddard Space Flight Center have developed a revolutionary process for precision optical polishing of bare aluminum to an unprecedented smoothness. GSFC's process begins by using a single-point diamond turning machine. Grinding cannot be used on bare aluminum, it leaves behind particles that scratch the surface during polishing. Diamond turning alone, typically produces a 30- to 80-angstrom finish on standard aluminum materials. Therefore, additional polishing is required to achieve the needed smoothness for low-scatter, high-quality bare aluminum optics. GSFC's process uses a special compound to polish the mirror to a super smooth finish. This compound not only offers superb lubricating qualities, but it also contains suspended particles. These particles are extremely hard and small, making the compound ideal for mirror polishing. Flat and spherical mirrors polished with GSFC's process have a roughness of 5 angstroms rms while maintaining a surface figure accuracy of 0.125 of a wave peak to valley. Aspherical mirrors can be polished to a 10-angstrom rms finish. The major benefit of this innovative process is the ability to make pure aluminum mirrors. Aluminum optics are less expensive and lighter than optics made from other pure materials. Also, nickel plating aluminum optics has drawbacks. Plating faults can ruin the optic, and the bimetal thermal characteristics of nickel over aluminum are poor, which is problematic for space and other applications with dramatic temperature swings. Finally, the nickel plating process can be expensive. By enabling the fabrication of high-quality aluminum mirrors, GSFC's process can dramatically reduce component fabrication cost, while improving the performance of the optical system.
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