ESO’s Extremely Large Telescope reaches an important milestone

18 Dec 2023

An artist’s depiction of the Extremely Large Telescope. Image: ESO/L Calçada/ACe Consortium

When complete, the new telescope will be used to tackle the biggest astronomical challenges of our time.

Set to be up and running by the end of the decade, the European Southern Observatory’s (ESO) Extremely Large Telescope (ELT) is now one step closer to completion.

The first 18 segments of the telescope’s main mirror are on their way to the Atacama Desert in Chile where construction is taking place.

The mirror cannot be made in one piece and instead will consist of almost 800 individual segments arranged in a large hexagonal pattern, with an additional 133 being produced to facilitate the recoating of segments.

Once they arrive, the segments will be coated in preparation for their future installation on the telescope’s main structure. When the mirror is complete, it will be the largest telescope mirror in the world with a diameter of more than 39 metres.

The mirror segments were first cast by German company Schott. They were then delivered to French optical systems manufacturer Safran Reosc to be polished. As part of the process, the company developed new automation workflows and measurement techniques to ensure that the polishing met the high standards required for the telescope.

The surface irregularities of the mirror are less than 10 nanometres – less than one thousandth of the width of a human hair. To reach this level of performance, Safran Reosc used a technique called ion-beam figuring, in which a beam of ions sweeps the mirror surface and removes irregularities atom by atom.

Other companies involved in the mirror segments include a German-French consortium, which developed and manufactured the 4,500 nanometric-accuracy sensors monitoring the relative position of each segment, and Danish company DSV, which had the delicate task of transporting the segments.

Also known as ‘the world’s biggest eye on the sky’, the ELT was given the green light in 2006 before the site in the Chilean desert was selected in 2010. The site got its first foundations in 2018 and the telescope’s first light is expected in 2028.

Meanwhile, the ESO’s Very Large Telescope (VLT) has been used in many astronomical discoveries since its own first light in 1998.

In 2020 for example, observations made using the VLT helped reveal a stunning image of a spiral of dust and gas during the birth of a new star system. And just this year, it helped a team of scientists unravel the mystery around a pulsar that had been switching between two modes of energy emission in seconds.

Ireland has been a member of the ESO since 2018, meaning the country’s astronomers and physicists have much greater access to telescopes such as the VLT and, when built, the ELT. Earlier this month, Irishman Prof Tom Ray was announced as the next president of the ESO’s main governing body, leading the organisation as construction of the ELT continues.

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Jenny Darmody is the editor of Silicon Republic