Facing the light

SCHOTT manufactured a mirror substrate from the high-tech material ZERODUR® glass-ceramic for the world’s largest solar telescope

In mid-March, the US research organization AURA (Association of Universities for Research in Astronomy) accepted delivery of the most important component of its new solar telescope Daniel K. Inouye Solar (DKIST, formerly known with the project name ATST). In order to be able to better understand the complex processes on the sun, the $300 million telescope will begin reflecting its first light on Hawaii in 2019 after it has been polished and mounted.

Its 4.26-meter monolithic mirror made of ZERODUR® glass-ceramic will make the telescope the largest of its kind. As it gets pretty hot when you look into the sun and the imaging characteristics of the center mirror must not change, it is made of ZERODUR® glass-ceramic from SCHOTT, which has extremely low thermal expansion. 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 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. “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,“ says Dr. Thomas Westerhoff, Senior Manager Strategic Marketing of the ZERODUR® product group. „We will be able to use the technologies we developed to meet many more customer requests.”