The world’s largest telescope is made possible by this glass-ceramic from SCHOTT

SCHOTT has been awarded a contract for the production of mirror substrates for the secondary (M2) and tertiary mirrors (M3) of the Extremely Large Telescope. M2 will be the largest convex mirror ever produced. A second melting tank will soon go into operation to meet the sustained high demand for ZERODUR® glass-ceramic.
SCHOTT signed contract for mirror substrates for ELT
The largest optical telescope in the world – the Extremely Large Telescope (ELT) – is currently being built on the Chilean mountain Cerro Armazones. After shipping substrates for the telescope’s segmented fourth mirror last year, SCHOTT technology group will supply two additional mirror substrates made of ZERODUR® glass-ceramic, including the largest convex mirror ever produced.

ZERODUR® glass-ceramic is especially useful as a mirror substrate due to its extremely low thermal expansion coefficient. Even with large temperature fluctuations, the material does not expand. In addition, the material is chemically resistant and easily polished. The actual mirror layer is made of aluminum or silver, which is usually deposited with vapor shortly before the telescope is put into operation. This extremely smooth surface produces sharp images for exploring planets, stars, and galaxies.

In the ELT project of the European Southern Observatory (ESO), these features are especially effective because the design is ambitious: the 39.3-meter primary mirror, also called M1, collects light from the night sky and reflects it on the M2, which at four meters in diameter will be the largest convex mirror ever. M2 in turn reflects the light to a reflector (M3) placed in the center of the M1. The task of this third mirror is to irradiate the light onto adaptive optics (M4) placed above it, which changes its surface a thousand times a second to clarify images by compensating for atmospheric turbulence that can distort the picture.
“The ELT project is the kind of ground-breaking project ZERODUR® was engineered for,” said Christoph Fark, Executive Vice President Advanced Optics at SCHOTT. “ZERODUR® glass-ceramic is in high demand for IC/LCD lithography, aviation, and metrology, among other areas. To keep up with that demand, we will soon put a second melting tank into operation at SCHOTT’s main plant in Mainz, Germany. In addition, we have expanded the capacity of our CNC processing to grind the material in the two- and four-meter class by means of state-of-the-art CNC machines, which we will officially inaugurate shortly. The machines will help us further optimize our manufacturing capabilities and meet tight tolerances for applications with the highest demands, such as the mirror substrates for the ELT. In total, we’ve invested in ZERODUR® glass ceramic production in order to secure and further expand our excellent market position.”

Proven material – “Made in Germany”

The cast and the grind of the ELT mirror substrates will be carried out at SCHOTT headquarters in Mainz. The French company Reosc, which specializes in the processing of ZERODUR®, will take over polishing and coating. The mirror substrate of the M2 is to be produced and delivered by the end of 2018, and the M3 by July 2019.

SCHOTT recently launched a microsite ( where interested people can find information around the ELT project and new telescope technology. The content of the website will grow over the coming months, providing facts and figures around the mega telescope and illuminating SCHOTT’s contribution.

Knowledge from research and development translates into other products

Since 1903, SCHOTT has been providing large telescopic mirror substrates, and since 1970, SCHOTT ZERODUR® has established itself as the de facto standard. Accordingly, many well-known telescopes have been operating reliably for decades with ZERODUR® glass ceramic mirror substrates. This includes, for example, the ESO’s Very Large Telescope in Chile with four mirror substrates, each measuring 8.2 meters in diameter.

SCHOTT has already translated the technology behind ZERODUR® into other applications. The high-quality glass-ceramic cooktop panel SCHOTT CERAN® and SCHOTT ROBAX® transparent glass-ceramic for fireplaces are two examples of materials based on ZERODUR®. The low thermal expansion coefficient allows ROBAX to convincingly withstand high temperatures for a greater degree of fireplace safety in the more than 75 million fireplaces that have been sold over the last 35 years.
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