The Extremely Large Telescope

The Extremely Large Telescope
At 39 metres in diameter, the mirror of Europe’s Extremely Large Telescope (ELT) will be the largest in the world. Scientists plan to begin serious observations with it around 2025, from a mountain-top site 10,000 feet high in Chile.

Unlike the 5-metre (200-inch) Hale telescope in California (which was the largest in the world when built in the 1950s), the ELT will use not a single mirror but 798 smaller segments. Modern computer technology allows the light from all these mirrors to be combined into a usable image. Central to the $1.5 billion telescope are these mirrors, which are made of Zerodur.

This year marks the 50th anniversary of the first order for Zerodur, which was manufactured and patented by the Schott glass company of Mainz, Germany in 1967. That initial order came from the Max Planck Institute, and there are two amusing versions of how it happened.

According to people at Schott, they went to the office of Prof. Elsaesser at the Institute with samples of Zerodur. Elsaesser was asked if he would order enough to cast a 4-metre blank. He answered, “Yes, of course.” Colleagues who knew the professor have a slightly different version of the tale. They say representatives from Schott put a handful of Zerodur on the table and said “Where is the order for the 4-metre blank?” Whether Prof. Petzold, the material developer of Zerodur, was present is not known.

Two other very large telescopes are under construction. There are plans afoot to use Zerodur in the secondary mirrors of the Magellan telescope, whose primary mirror is made of borosilicate glass (similar to the pyrex most people are familiar with). And for the Thirty Metre Telescope (TMT), both the secondary and tertiary mirrors are being made of Zerodur.

One of the great benefits it offers is an extremely low thermal expansion, obviating the need for a temperature controlled mirror being used for the Magellan. The optics for the ELT are being manufactured by Safran, a company based in France that was founded 80 years ago.

Schott has a booth in the exhibitor room at the international telescope conference underway at the Austin Convention Center. Earlier today (June 13) ELT manager Dr. Roberto Tamai gave an update on the ELT. The first ceremony to mark the opening of the site was held by the President of Chile on May 26, 2017. This was the same month Schott announced it would build the mirrors.

More than 90% of the material cost has been committed, and more than 30% of the large contracts are underway. Work is progressing on all the prime instruments it will use, and as of this month funding has been approved for a 2nd generation of instruments. Construction of the technical facility in Chile will begin in August.

Sun News Austin will report further on the TMT as a Hawaii Supreme Court hearing on its future is being held on June 21, 2018.

To learn more about Schott (founded in Jena in 1884) and the ELT, visit the website:

Image: artist’s conception of what the ELT will look like when complete (credit: ESO/ L. Calcada)

By Dave

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