”Sunrise” over Arctic Ice

The largest solar telescope to ever leave Earth flew across the northern Arctic Circle with a balloon in the middle of 2009 – equipped with a lightweight mirror made of Zerodur® glass-ceramic.

On the morning of June 8, 2009, a special balloon from NASA slowly ascended into the blue sky above the Esrange Space Center, near Kiruna in the north of Sweden. The largest solar telescope ever to leave Earth was inside its gondola. The goal of ”Sunrise”, a science project initiated by the Max Planck Institute for Solar System Research (MPS), is to deliver the highest quality images of our sun ever to be taken.

Carried by a helium-filled balloon as large as
the famous Frauenkirche in Dresden, Germany, when it unfolds at an altitude of around 37 km, ”Sunrise” kept its eye on the sun that shines here around the clock in the summer. For six days, the polar winds blew the solar observatory through the stratosphere over the North Atlantic and Greenland all the way to Canada. On June 14, ”Sunrise” finally landed by parachute on Somerset Island, a large island near the Northwest Passage.

”Although the wind was blowing quite strong when it landed, all of the main components, even the telescope’s primary mir‑​ror, remained undamaged,” notes Professor Sami K. Solanki, ­Director of the solar department at the MPS. ”All of the instruments on board Sunrise functioned just perfectly, therefore the fine structures and magnetic fields of the sun could be measured with high precision,” he adds. This data is being processed and analyzed.

The telescope that features a primary mirror 1.1 meters in diameter is the heart of ”Sunrise”. Thanks to the filigree lightweight structure of the backside of the mirror, the monolith made of  Zerodur® glass-ceramic from SCHOTT is a real lightweight at 45 kilograms. Although some of the partition walls contained in the triangular honeycomb structure are only four millimeters thick, the mirror substrate remains extremely precise and stable.

”Zerodur® offers one outstanding characteristic in that its coefficient of thermal expansion is close to zero. The glass-ceramic is extremely stable and, with a specific density of 2.53 grams per cubic meter, even lighter than aluminum,” explains Dr. Thomas Westerhoff, head of the product divison Zerodur® at SCHOTT. ”By using the right type of design and modifying the machining processes, it might even be possible to reduce the weight by more than 90 percent,” he notes.

SCHOTT has been supplying large format Zerodur® mirror substrates for astronomical observations all over the world since 1968. With the Swedish Solar Telescope on La Palma, the New Solar Telescope at Big Bear Lake in California and the German GREGOR Telescope that will soon be put into operation on Tenerife, all of the world’s most modern solar observatories are currently equipped with mirrors made of Zerodur® glass-ceramic. Even larger solar telescopes are now in planning. Here, too, this material is already being considered – sunny prospects for glass-ceramics from SCHOTT.

Lightweight structured mirror blank made of SCHOTT Zerodur. Photo: SCHOTT

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Mirror blank made of SCHOTT Zerodur. Photo: SCHOTT

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”Sunrise” is designed to deliver images of our sun in previously unknown quality. A 1.1-meter primary mirror represents the heart of this telescope made of Zerodur® glass-ceramic from SCHOTT (Page 35, lower right). With its partial honeycomb structure that is only millimeters thick, it weighs only 45 kilograms. Photo: MPS

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