SCHOTT solutions no. 1/2010 > Astronomy

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.

Gerhard Samulat

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
Manufacturing this complex geometry called for the highest precision possible. ”Nevertheless, in some areas, we were even able to deliver greater accuracy of shape than we had committed ourselves to supplying,” Dr. Westerhoff notes. ”At certain positions, we were not allowed to deviate from these values by more than 40 micrometers,” he explains. This corresponds to about half the thickness of a hair. In addition, customers are demanding that their prized possession feature a rather sophisticated geometry. In order to provide sharp images of the sun, the mirror must have an aspherical shape. This means the surface needs to have a different curvature radius in the middle than along the outside rims. “We did a much better job on the envelope than the order called for,” Dr. Westerhoff notes. This made it easier for Carl Zeiss in Oberkochen to do the polishing. ”Nevertheless, the greatest challenge was to involve both our personnel and technology in the manufacturing process as effectively as possible,” adds Thomas Werner, the production manager responsible. During the three-month processing phase, more than 450 geometric characteristics with tight tolerances were produced without any defects. GREGOR is equipped with adaptive optics that compensate for the effects of unsteady weather in Europe’s atmosphere on the image of the sun, which would otherwise be unclear. The astronomers are now able to observe even the smallest structures only 50 kilometers in size on the surface of the sun, which is roughly 150 million kilometers away. As soon as the telescope that is being built by the Kiepenheuer Institute of Solar Physics in Freiburg, the Astrophysical Institute of Potsdam, the Institute for Astrophysics in Göttingen and the Max Planck Institute for Solar System Research has been completed, its instruments will enable it to measure many of the features of the central star with unprecedented precision. This is particularly true for its variable magnetic field that is said to be responsible for sunspots, as well as outbreaks of large amounts of solar material. When a shower of particles from these eruptions hits the earth, they not only cause the fascinating polar lights, but can also destroy the electronics in satellites or cause power failures and interrupt telecommunications. To make sure this doesn’t take us by surprise, GREGOR will continue to keep a close eye on what is happening up above us.