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.
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
(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
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
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