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A sharp view of space

In order for the world’s largest telescope to provide a clear view of the stars, the system’s internal mirror assembly must retain its shape de- spite temperature changes and rapid movements. This is achieved through the use of mirrors made with a new combination of materials.

Perched atop a lonely mountain peak in southwest Arizona, USA, the Large Binocular Telescope (LBT) – the world’s largest telescope – is a joint project between the USA, Italy and Germany. Two mirrors measuring eight meters in diameter reach the sensitivity of a 12-meter telescope and the resolution of a 23-meter telescope. The piston mirror, an assembly consisting of two additional mirrors, each with a diameter of about 20 centimeters, also contributes to the telescope’s performance by merging the two optical beams in the detector and equalizing phase shifts in the ray beam. The optics package must meet two requirements: its mass must be kept as small as possible so that it can be easily and rapidly moved within a few micrometers; and the surfaces of the mirrors must hold their shape when subjected to temperature variations and acceleration forces. Researchers at the Max Planck Institute für Astronomie in Heidelberg worked with associates from the Fraunhofer Institute for Applied Optics and Precision Engineering IOF in Jena to find a mirror material that meets these conditions. “It was clear from the beginning that the material we were looking for would be coated with ‘chemical nickel’,”explains IOF project manager Andreas Gebhardt. “The sought-after material therefore had to expand and contract in the same manner as the nickel coating when subjected to temperature fluctuations. Otherwise, dissimilar expansion behavior in the materials would warp the mirrors and impair the image quality.” Conventional materials such as aluminum either have a considerably higher expansion coefficient than nickel or are very expensive. A recently developed silicon-aluminum alloy adapted for expandability, on the other hand, exhibits properties similar to chemical nickel and is also comparatively economical. This alloy also fulfills the second requirement: its relatively high degree of rigidity makes it ideal to manufacture very stable low-weight structures. The tests conducted thus far confirm the suitability of the material. Combining this alloy with nickel in the piston mirror represents the first such application for an optical component – an innovation that will give the LBT an especially sharp view of objects in the univers beginning in 2008.For further information: Andreas Gebhardt Phone: +49 36 41 / 8 07-3 40 andreas.gebhardt@iof.fraunhofer.de Picture in color and printing quality: www.fraunhofer.de/press

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