SCHOTT solutions no. 1/2014 > Astronomy

Only with the DKIST will it be possible to collect precise polarimetric data with the temporal cadence necessary to capture the evolution of the fine structure of sunspots and finally understand its physical origin. Source: NSO/AURA/NSF/T. Rimmele

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.
The world’s largest solar telescope is scheduled to be put into operation in 2019 on the 3,000-meter high mountain Haleakalã on the Hawaiian island of Maui. Photo: Brett Simison/NSO/AURA/NSF

Solar telescope of superlatives

Its 4.26-meter monolithic mirror made of ZERODUR® glass-ceramic will make the telescope the largest of its kind. The huge mirror diameter will allow for structures 15.5 miles in size to be observed on the sun which is nearly 93 million miles away. That can be compared to viewing a pea located 18.6 miles away. The resolution is thus a factor of 2.6 higher than what was achieved with the largest solar telescope at Big Bear Solar Observatory near Los Angeles. The researchers are particularly interested in the processes on the sun which result in flares and  coronal mass ejection. In addition they want to develop capabilities for space weather predictions. They hope these processes will provide them with insights that can also be of use in fusion reactions here on Earth. The large mirror aperture also allows for better temporal resolution and observation of narrow spectral regions, especially in the infrared range of the solar spectrum, which has hardly been researched to date.
The DKIST – here as a rendering – features a large mirror opening that allows for better spatial resolution and observance of narrow spectral regions, particularly in the infrared range. Source: LeEllen Phelps/NSO/AURA/NSF
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.
Manufacturing the mirror substrate posed a major challenge for the engineers at SCHOTT with respect to both the extreme demands placed on the melting (see picture above) and processing it into an off-axis asphere. Photo: SCHOTT/C. Costard
The properties of ZERODUR® glass-ceramic are only one aspect; in addition the specifications the scientists had established for the mirror material posed a major challenge. ”Since the 8-meter project that required manufacturing four primary mirrors that were each 8.2 meters in diameter, the largest monolithic mirror substrates ever cast, no other job has challenged us and helped us to achieve advances in terms of technology as much as this project. We will be able to use the technologies we developed to meet many more customer requests,” Project Head at SCHOTT Thomas Werner explains. The glass-ceramic needs to be extremely homogeneous; for example, because bubbles and inclusions would result in scattered light that reduces the contrast. SCHOTT has succeeded in manufacturing a mirror substrate in which the maximum number of bubbles per unit volume was one order of magnitude lower and the bubble size permitted in the critical layer was undercut by a factor of 2.5.
Thanks to the 4.26-meter mirror substrate made of ZERODUR® glass-ceramic, the DKIST will be the world’s largest solar telescope. This monolith, which weighs nearly three tons, is only 7.5 centimeters thick (see picture above). Photo: SCHOTT/C. Costard
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. <
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