The largest telescope on Earth starts with 1,400-degree molten glass

When completed, the Extremely Large Telescope (ELT) will be the largest ground-based optical telescope in the world for visible and near-infrared light.

Scientists hope it can image exoplanets outside our solar system, uncover clues to how galaxies formed, and provide a window into the physics behind the expansion of our universe. In short, the ELT will be, in the words of one European Southern Observatory (ESO) official, “humanity’s largest eye on the sky.”

And it all starts in a 1,400-degree Celsius tank of molten glass.


hot molten glass with a stickThe time has finally come. SCHOTT has begun casting the first of 949 mirror blanks for the main mirror, in a highly technical operation that will take years to complete.

To make the panels, liquid glass is heated to 1,400 degrees Celsius, and poured into specially designed molds.ELT Zerodur glass melt

extra large telescope zerodur



SCHOTT ELT castFrom there, the molds are taken to a cooling furnace, and subjected to a process to transform the molten glass into glass-ceramics. After ceramization, each of the round blocks will be cut into five slices of approximately 60 – 70 mm in thickness.

Further processing at SCHOTT will take place on state-of-the-art, computer-controlled 5-axis CNC machines. SAFRAN Reosc, a French company with broad expertise in defense, optics, and engineering, will polish the segments into an extremely smooth final product.

This particular formulation of glass-ceramic is called ZERODUR®, and it’s been a preferred material for space and Earth-based telescopes for 50 years. ZERODUR® has flown successfully on over 30 space missions, including the Chandra X-Ray Observatory, the secondary mirror of the Hubble Space Telescope, and both NASA Great Observatories. It can also be found in some of the most significant terrestrial telescopes, including the ESO’s Very Large Telescope, and the Daniel K. Inouye Solar Telescope.

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Hi, I’m Steve Sokach. I’ve been with SCHOTT for 27 years and currently serve as the Director of Sales for the Advanced Optics Business Unit in the Americas. I received my bachelor’s degree in Optics from the University of Rochester and an MBA from the University of Scranton. On a personal note, I enjoy supporting my three daughters’ academic and athletic activities and traveling with my wife.

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