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17 Jun 2002
NASA's Next Generation Space Telescope (NGST), scheduled for launch in 2008, will include 4 million mirrors, each the size of a human hair and able to move independently. The mirrors will be sensitive to infrared radiation and, as a result, will be able to detect faint signals from the early universe.
Prototypes of these MEMS mirrors have been developed by Ernie Garcia and colleagues at Sandia National Laboratory in the US as part of a year-long contract for NASA.
The prototype is an array of mirrors, each 100 micrometres by 100 micrometres with one-micrometre gaps between adjacent mirrors. Each row tilts in unison by 10 degrees. The goal is to have each mirror tilting in different directions to redirect optical signals to an infrared detector.
The micromirrors will work in conjunction with a very large mirror which will collect light from a broad area in space. When an interesting object is discovered, the smaller micromirrors will tilt to reflect the image from that area towards an infrared detector.
The Sandia prototypes are made by depositing thin films of polycrystalline silicon onto a silicon wafer. The first layer contains connection wires while the others are mechanical layers that move the MEMS device. A thin layer of gold will be added on top to reflect infrared light.
A major barrier to the development of these mirrors is the operating temperature in space, which can be 30 degrees Kelvin or lower. As the temperature is reduced, the gold layer will shrink faster than the polysilicon, causing stress, which could break or deform the mirror or make the gold could peel away. To reduce the stress, the gold layer must be as thin as possible without hindering the reflection of the mirror.
The NGST will orbit the Earth well beyond the moon to reduce stray light and achieve the cold temperatures needed for infrared observations. It is designed to replace the Hubble Telescope, which observes visible light.
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