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Watch the Largest Convex Mirror Ever Get Made

The secondary mirror for the Extremely Large Telescope will take a year to cool down.

by Daniel Oberhaus
May 24 2017, 6:41pm

On Monday, the European South Observatory announced that it had successfully cast the largest convex mirror ever made. This massive mirror—which is nearly 14 feet across and weighs 3.5 tons—will be used in the Extremely Large Telescope (ELT) in Chile. As its name suggests, the ELT will be the largest optical telescope ever built when it achieves first light in 2024 after a decade of construction.

The secondary mirror marks a major milestone in the process of getting the ELT up and running. It is made from a proprietary ceramic-glass material called Zerodur and was pressed by Schott, the same German manufacturing group that made the mirrors for the European South Observatory's Very Large Telescope.


For the next year, the glowing hot mirror will be stored in Germany. After it has cooled, it will transported to France where it will be ground to the right shape and precisely polished so that there is never more than a 15 nanometer difference across the mirror's entire surface. Finally, it will be suspended upside down over the massive ELT primary mirror, which is 128 feet in diameter.

Artist rendering of the Extremely Large Telescope. Image: ESO

In the meantime, however, there is still work to be done. The ELT will use five mirrors in total, but so far only the quaternary mirror has been completed. The primary mirror will consist of a nearly 800 hexagonal mirror segments and the tertiary mirror will have a curved surface, an unusual design feature in modern telescopes. Once completed, the telescope will be used to study exoplanets, the formation of galaxies in the early universe and super-massive black holes in unprecedented detail.

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