When particles that result from solar eruptions land on earth, they create polar lights. But they can also cause power failures or disturb telecommunications. To make sure this doesn’t take us by surprise, the solar telescope GREGOR will be keeping a watchful eye on the sky above us. Photo: Kiepenheuer Institut
GREGOR the Great
Solar telescopes require mirrors that offer exceptionally high temperature stability. Today’s most advanced models use Zerodur® glass-ceramic.
The astronomers at the Kiepenheuer Institute of Solar Physics in Freiburg, Germany, eagerly awaited the arrival of the most important component of their new solar telescope GREGOR: the primary mirror 1.5 meters in diameter made of Zerodur® glass-ceramic, whose weight was reduced through laborious processing. All of the other components of the observatory built near Spain’s highest mountain, Teide on the Canary Island of Tenerife, have already been finished and checked by the astronomers using a test mirror. The world’s highest performance solar telescopes already use this glassceramic from SCHOTT that is easy to process and offers extremely high temperature stability, for instance the Big Bear Solar Observatory BBSO in California, the Swedish solar telescope SST which, like GREGOR, is based on a Canary Island, and Sunrise, a telescope that lifted off above the polar circle in the Arctic in a balloon in mid-2009. The Zerodur® material also ranks as the ”hottest” candidate for the next generation of solar telescopes, the American Advanced Technology Solar Telescope ATST on Hawaii, as well as the European Solar Telescope EST that will also be built on Tenerife. Although the mirror substrates are exposed to direct solar radiation, they hardly expand at all with heat: ”We once calculated that the length of a piece of Zerodur® that is ten kilometers long would change by only about one centimeter between the summer and the winter,” says Dr. Thomas Westerhoff, head of the Zerodur® product group at SCHOTT.
The Zerodur® mirror substrate – shown here while it is being polished – features a complex pocket structure on the back side that helps achieve the right degree of weight reduction. Photo: Carl Zeiss
ZERODUR® Glass Ceramic
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