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Distortion-free mirror takes flight

13 Jul 2007

A mirror that maintains its shape to provide a constant optical image at extreme temperatures will be flying high in the world's largest airborne observatory.

A mirror that maintains its shape under large fluctuations in temperature is to be housed on the world's largest airborne observatory. The mirror will collect light and infrared radiation 12 km above the Earth in temperatures as cold as -60 °C.

The glass ceramic mirror, made from a material called Zerodur, has been manufactured by German technology company Schott. "The material demonstrates a nearly zero thermal expansion coefficient at ambient temperatures between 0 °C and 50 °C," Mary Frances Scott of SCHOTT told optics.org.

It will form the 2.7 m diameter, 0.35 m thick primary mirror on SOFIA (Stratospheric Observatory For Infrared Astronomy). SOFIA is a joint venture between NASA and the German Aerospace Center (DLR) that is expected to reveal new insights into the formation of stars, star clusters and galaxies.

Zerodur is already used in some of the largest telescopes in the world, such as the Very Large Telescope in Chile and the 10 m Keck telescopes in Hawaii, but the material has had to be adapted to reduce the weight for the airborne telescope.

"A honeycomb structure is used on the backside of the mirror blank, which has no effect on the reflectivity, but reduces weight and therefore reduces fuel and environmental costs," explained Scott. "Approximately 75 percent of its weight comes from tiny micro-crystallites embedded in a glassy phase. These micro-crystals contract when they are subjected to heat, whereas the glass itself expands. The size and number of the micro-crystallites are adjusted to achieve an extremely low thermal expansion."

Zerodur is an inorganic, non-porous glass ceramic that contains lithium, aluminum and silica as its main chemical components. Production takes many months to prevent mechanical stresses within the glass. The chemical ingredients are first melted at temperatures of more than 1500 °C before being slow-cooled to room temperature within a mold.

The cooled glass is then cut and ground to form the final specified shape. It is then heated to around 1000 °C for several months to transform it into a glass ceramic with zero expansion coefficient, followed by another period of controlled slow cooling.

Telescopes are not the only applications that use the material. "Zerodur can be used wherever a high degree of accuracy in shape and distance must not be affected by changes in temperature," explained Scott. "For example, the material is also used in high precision mechanical parts, optical elements for lithography equipment, and as standards for precision measurement technology.

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