Sights Set on the Sun

SCHOTT manufactured a mirror substrate from the high-tech material ZERODUR^® glass-ceramic for the world’s largest solar telescope DKIST (Daniel K. Inouye Solar Telescope). The ”hot gaze” at our central star poses special challenges.

Bernhard Gerl

All life on earth owes its existence to the sun. But besides providing us with light and heat, the sun occasionally also ejects heavy particle showers toward Earth together with the solar winds that threaten our sensitive electronics, satellites, and even our power grids. That’s why scientists are trying to better understand the complex processes on its surface. To this end, the $300 million Daniel K. Inouye Solar Telescope (DKIST, formerly known as ATST) will be put into operation in 2019 on the 3,000-meter high mountain Haleakalã on the Hawaiian island of Maui. The National Solar Observatory is responsible for the research that the U.S. umbrella organization AURA (Association of Universities for Research in Astronomy, Inc.) is conducting and is supported by 22 research institutes from many different countries.

In contrast to night-telescopes that greedily collect every photon, solar telescopes struggle with the opposite. It gets pretty hot when you look into the sun. To ensure that the imaging characteristics of the center mirror do not change, it is made of ZERODUR^® glass-ceramic from SCHOTT, which has extremely low thermal expansion. “The success of this material thus continues, because with the Swedish Solar Telescope (mirror diameter 1 m) on La Palma, the New Solar Telescope on Big Bear Lake in California
(1.6 m), the ”Sunrise” that is carried by a balloon (1.1 m), and the German GREGOR Telescope on Tenerife (1.5 m), all of the world’s largest and most modern solar observatories are currently equipped with mirror substrates made of ZERODUR^® glass-ceramic,” explains Dr. Thomas Westerhoff, Senior Manager Strategic Marketing for the ZERODUR^® product group.

The scientists sought to achieve an expansion coefficient in the range of
± 30∙10^– 9 per Kelvin, however SCHOTT actually even achieved a level of +6∙10^– 9 per Kelvin with a deviation of only 3∙10^– 9 per Kelvin for the entire material. A piece of ZERODUR^® glass-­ceramic 100 miles in length would only expand by 0.4 inch if the temperature increased by 100 Kelvin.

The mirrors used in solar telescopes must meet extremely high mechanical demands because they gaze directly at the rising sun and therefore must be vertical. The DKIST mirror is only 2.9 inches thick so that it is easy to cool from behind, yet bears nearly three tons of weight without changing its shape. It is supported by 120 actuators on its back, which compensate for the deflection that inevitably occurs.

The special design of the telescope also had consequences for the mirror. Since no other secondary mirrors that cast shadows were to be located within the optical path in the telescope, the mirror had to be ground in the form of an off-axis asphere, which means that the surface in the middle has a different radius of curvature than along the outer edges and that this “center” is also not located in the geometric center of the mirror. As a consequence of this grinding, however, the focal point does not lie above the mirror, but rather light is reflected to the side instead. Achieving this unusual shape also required a great deal of effort. Nevertheless, it was worth it, considering how satisfied the customer is. As DKIST Project Manager Joseph McMullin expresses, ”Manufacturing a mirror substrate with the necessary specifications clearly turned out to be a huge technological challenge. Here, SCHOTT proved to be the perfect partner for our project. They really did a great job.” And so it was that the mirror left Mainz for Bremerhaven as a heavy load shipment on its roughly six-week journey at the end of January. From there, it was put on a ship to cross the Atlantic and head for the Panama Canal and from there to Los Angeles. In order to survive the often rough waves while crossing the Atlantic and the drive to Tucson, Arizona, it was stored on a special shock-absorbing system. According to the current plan, once it has been polished and successfully installed, the mirror will begin reflecting its first light to the instruments in 2019 so that the sun can be observed more closely. <