SCHOTT solutions no. 1/2016 > Optical Mirrors

Optical Mirrors
The Hobby-Eberly Telescope in Austin, Texas, is equipped with an 11-meter mirror made of 91 SCHOTT ZERODUR® elements. The Dark Energy Experiment also uses BOROFLOAT® mirrors in 150 spectrographs. Photo: Ethan Tweedie Photography

Enlightening the darkness of the cosmos


Optical mirrors made of BOROFLOAT® specialty glass installed in the Hobby Eberly Telescope help to solve one of the greatest mysteries of our universe: ”Dark Energy.”


Christiane Gallo

According to scientific research, normal matter of stars, planets and living beings makes up only four percent of the total mass and energy of the cosmos. So-called “dark matter,” which is responsible for the gravitational pull and cohesion of galaxies and galaxy clusters, accounts for nearly 21 percent. ”Dark energy,” however, contributes the lion’s share of over 74 percent. Many cosmologists share the theory that it is responsible for the faster expansion of our universe – in spite of the opposing force of matter. Nevertheless, no evidence has yet been presented that dark energy even exists. Researchers are trying to support their theories with the help of powerful particle accelerators or space telescopes.
Optical Mirrors
Left: Model of the upgraded Hobby-Eberly Telescope. The 150 spectrographs are contained in the curved gray ”saddlebags” on the side of the telescope. Photo: McDonald Observatory/HETDEX Collaboration Right: ”BOROFLOAT® contributes significantly to the fact that researchers can take a look back at the earliest millennia to better understand the forces of the universe,” says Dan Bukaty Jr., President of Precision Glass and Optics Inc. Photo: SCHOTT
One of these projects is based in the US state of Texas. HEDTEX, short for Hobby-Eberly Telescope Dark Energy Experiment, is located at the McDonald Observatory at the University of Texas in Austin. The telescope takes a deep look into the past of the universe with 150 ultra-modern spectrographs in order to calculate the three-dimensional position of a million galaxies. The optical instruments capture the entire light spectrum in order to determine how far the galaxies were away from one another at different times and to reveal their composition. These measurements will hopefully help in investigating the expansion rate of the cosmos and provide us with a better understanding of the physics of dark energy.

The spectrographs rely on optical mirrors made of BOROFLOAT® special glass. The floated borosilicate glass from SCHOTT was chosen because of its exceptional properties. Its low thermal expansion coefficient helps to ensure consistency of the spectrograph’s high resolution and its measurement results. In addition, the special glass offers high chemical resistance and excellent mechanical strength due to its high content of boron oxide. The low refraction index in combination with excellent light transmission are key requirements for reliable test results from the spectrograph.

”The special glass contributes significantly to the fact that researchers can take a look back at the earliest millennia to better understand the forces of the universe,” says Dan Bukaty Jr., President of Precision Glass and Optics Inc., the US company that processes BOROFLOAT® special glass into optical mirrors. <