Cool gaze, hot view

SCHOTT manufactures mirror substrates from “Zerodur” for the world’s largest solar telescope ATST

Maui (U.S.A.) / Mainz (Germany), June 15, 2010 – The Association of Universities for Research in Astronomy (AURA) has selected SCHOTT to build the 4-meter primary mirror blank for Advanced Technology Solar Telescope (ATST), which will be the world’s largest solar telescope. To obtain ultra-sharp images of the flaming celestial sphere, researchers need a cool and undistorted view. The primary mirror substrate that is as thin as 76 millimeters and 4.25 meters in diameter will be made out of “Zerodur” from SCHOTT. The glass-ceramic with zero thermal expansion already delivers exact measurement data and precise images in solar telescopes like GREGOR, Sunrise and Big Bear New Solar Telescope.

Astronomers plan to build the world’s largest solar telescope on Haleakalā, Maui, Hawai’i. The air is exceptionally clear, dry and still here, in the middle of the Pacific, more than 3000 m above sea level – ideal conditions for a terrestrial telescope to observe the full scope of electromagnetic activities on the surface of the sun. The project is funded by the U.S. National Science Foundation.

Construction of the ATST is scheduled to begin following the permitting process of the Hawaiian Department of Land and Natural Resources with operation anticipated to commence in 2018. The primary mirror has a diameter of 4.25 meters, which will be masked to a clear aperture of 4 meters, and focuses the incoming sunlight through a heat stop then into the telescope’s optical system.

The curved primary mirror substrate will be made of the “Zerodur” glass-ceramic from SCHOTT and is approximately 76 millimeters in thickness at its thinnest point. This allows the mirror to be easily adjusted using the actuators on its backside, but also to cool it to ambient temperature to reduce image distortion.

“Our glass-ceramic features the outstanding characteristic of having a coefficient of thermal expansion that is close to zero,” explains Dr. Thomas Westerhoff, head of the “Zerodur” product group at SCHOTT. “This allows for light to be reflected without being distorted. Besides, this glass-ceramic is extremely stable with respect to its shape and, with a specific density of 2.53 grams per cubic centimeter, even lighter than aluminum,” he notes.

SCHOTT has been supplying large-format mirror substrates made of “Zerodur” to astronomical observatories all over the world since 1968. With the Swedish Solar Telescope located on La Palma, the New Solar Telescope on Big Bear Lake in California, the “Sunrise” telescope carried by a balloon and the German telescope GREGOR on Tenerife that will go into operation soon, the world’s most modern and largest solar observatories are all equipped with mirror substrates made of “Zerodur” glass-ceramic.

Casting of a four-meter mirror substrate with the “Zerodur” glass-ceramic at SCHOTT in Mainz (Germany). The molten glass that has a temperature that exceeds 1500 degrees Celsius is allowed to cool down, and then preprocessed and ceramized. Only then does this glass become a glass-ceramic. Source: SCHOTT

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Quality control of a “Zerodur” glass-ceramic disc. Source: SCHOTT

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Primary mirror of the ATST: the curved mirror substrate made of the “Zerodur” glass-ceramic can be adjusted to suit the surrounding conditions using actuators and controlled ventilation on its backside. Source: AURA ©2010, used with permission.

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The Advanced Technology Solar Telescope (ATST) is being built on the Hawaiian volcano Haleakalā (“House of the Sun”). The “first light” is scheduled for 2018. Source: AURA ©2010, used with permission.

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SCHOTT is an international technology group that sees its core purpose as the lasting improvement of living and working conditions. To this end, the company has been developing special materials, components and systems for 125 years. The main areas of focus are the household appliances industry, pharmaceuticals, solar energy, electronics, optics and the automotive industry. The SCHOTT Group is present in close proximity to its customers with production and sales companies in all its major markets. The Group’s approximately 17,300 employees generated worldwide sales of approximately 2.2 billion Euros in the fiscal year 2007/2008. The company’s technological and economic expertise is closely linked with its social and ecological responsibility. SCHOTT AG is an affiliate of the Carl-Zeiss-Stiftung (Foundation).

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