Original Date: 04/26/1999
Revision Date: 01/18/2007
Best Practice : Adaptive Optics Mirror Systems
The very large space telescopes under study at the NASA Marshall Space Flight Center (MSFC) involve large- diameter mirrors. Disadvantages of conventional monolithic designs included reliance on large, expensive, power- demanding electronics for future sensing and actuation; unnecessary risk of damaging the mirrors due to difficulty in handling them; and the need for large fabrication and test facilities. Floating point gate arrays are used for flexible, broad application, miniaturized electronics. Conventional labor-intensive grinding and polishing operations often induce unacceptably large stress fields in the material. To resolve these drawbacks, MSFC’s Advanced Optics Development Group is developing mirror systems that use ultra lightweight replicated mirrors.
The redesigned mirror system has many improvements over its predecessor. Segmented mirrors now exist as hexagonal tiles with diameters of less than one meter, compared to the previous 1.5 to 3.5-meter monolith designs. These smaller mirrors feature thinner cross-sections, less mass, and easier, cost effective fabrication. MSFC’s system also uses modular electronics with an extensible architecture for growth paths; a serial data loop originating at the master timing module; and symmetrical card and connector layouts. Modular electronics exist in seven, 36, and 91 segments with an application currently underway at a major observatory. Instead of using surface mount components, floating point gate arrays employ flexible and programmable devices (e.g., ROMs, PROMs). As a result, these arrays operate more efficiently and increase the bandwidth from nearly 0 Hertz (Hz) to more than 100 Hz. Replicated mirrors are now fashioned by depositing mass where needed, rather than by removing excess via mechanical means. This approach lowers internal stress fields, improves mirror figures, and provides high quality products at less than $20,000. Additional enhancements result from the electro-mechanical deposition on mandrel with sub-nanometer micro-roughness and diffraction-limited figure.
With these design changes, the Advanced Optics Development Group significantly revised optics mirror systems for space telescopes and gained valuable knowledge for future improvements. The Group determined that the optimum hexagon size is between seven centimeters and one meter; high quality test bed emulators become crucial during the development phase; new-generation floating point gate arrays continue to enhance the product design paradigm; and the tight radius of curvature specification exceeds the capability of commercial sources of replicated mirror segments.
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