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Made from molten glass, world’s largest convex mirror will take a year to cool

ESOcast 107 Light: Secondary Mirror of ELT Successfully Cast
Casting of the secondary mirror for the European Southern Observatory’s Extremely Large Telescope has been completed in Mainz, Germany.

Weighing 3.5 tons and measuring 14 feet in diameter, the finished piece is the largest convex mirror ever produced.

“This secondary mirror is one of several that focus the light coming into the ELT and help to create a very sharp final image,” Marc Cayrel, lead engineer for optics production, told Digital Trends. “It will sit at the top of the telescope, high above the main mirror, with its reflective surface facing down. It is exciting as it is the largest such secondary mirror ever made for an optical telescope.”

The giant convex mirror was created using a ceramic-glass material called Zerodur. It was made by Schott, a German glass-making company which also made the mirrors for the ESO’s previous Very Large Telescope.

As you might imagine, creating a convex mirror weighing several tons isn’t exactly easy. “Convex mirrors like this are much harder to shape and test than concave ones,” Cayrel continued. “The ELT secondary is also significantly different from a spherical surface, which again makes it harder to make. Up to now the largest convex mirrors did not exceed 1.4 metres (read: 4.5 feet) in diameter.”


Don’t think that the hard work is over yet, either. Now that the mirror’s been cast, the team must wait more than year for it to cool down from its casting temperature of 2,552 degree Fahrenheit, to the point where it can be safely used.

“The full process is in several stages and takes more than a year, it has to cool slowly to avoid the creation of stresses in the glass ceramic material,” Cayrel said.

First of all, the mold is placed in an oven that will control the cool-down over a three-month period. This allows for the minimizing of stress. After that, the mirror will be heat-treated to transform the glassy part into a ceramic, which is a process that will last another six months. Finally the M2 meniscus shape will be extracted by machining.

Still, it will all be worth it when the Extremely Large Telescope — the largest optical telescope on Earth — is completed in 2024!

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