Path to Meter Class Single Crystal Silicon (SCSi) Space Optics
Douglas R. McCarter
McCarter Machine, Inc, dba McCarter Technology, Inc, Deer Park, TX
With the global financial crisis affecting funding for space systems development, customers are calling for lower cost systems. Yet, at the same time, these lower cost systems must have increased thermal response to operational environments and increased ability to survive loads. A common misconception is that base substrate materials do not matter because coatings compensate for the inferior materials. Even on the surface, this is not true because collection of light is less efficient with a scattering finish. There are telescope materials that do not survive coatings because of internal stresses. There are some materials used that have to be misfigured at room temperature in hope that the cryo figure will meet specifications. There are materials that require costly and schedule consuming heating/cooling cycles to help reduce dimensional instability even at room temperature. There some materials whose properties change and/or dimensions when exposed to space radiation. There are materials that are hazardous to handle and exposed workers are subject to disease. Even ground based large telescopes can require full time focus management to survive small temperature fluctuations or vibratory loads. There are materials used in space where acquisition and/or spacecraft slew can create a ‘Lost in Space” condition. There are large space telescopes that are functioning at a limited mission goal and survival life. Therefore it should be clear that substrate material quality and behavior is of paramount importance. We have found a material that doesn’t succumb to these shortcomings - Single Crystal Silicon (SCSi).
SCSi can meet the challenges of lower cost and higher performance which can be grouped into two main areas – the material properties of SCSi and the processing methods used. The U S PATENT 6,443,817 Method of Finishing a Silicon Part allows for manufacturing induced stress and/or subsurface damage to be removed from silicon . Using glass-frit bonding. GFB, is the process of applying shattered glass powder to prepared silicon surfaces then fusing together using heat and pressure. These processes along with the material itself have been third party tested by universities and laboratories for reliability, performance, and survivability. Other testing has included mirror specimens for long term stability testing, fabrication of GFB lightweight mirrors for cryo use, and vibration test structures, all of which tested as dimensionally stable.
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