First six segments of Extremely Large Telescope cast

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The first six mirror segments of the European Southern Observatory’s Extremely Large Telescope have been successfully cast.

These segments will form parts of the ELT’s 39-metre main mirror, which will have 798 segments in total when completed. The ELT will be the largest optical telescope in the world when it sees first light in 2024.

The 39-metre-diameter primary mirror of ESO’s Extremely Large Telescope will be by far the largest ever made for an optical-infrared telescope. Such a giant is much too large to be made from a single piece of glass, so it will consist of 798 individual hexagonal segments, each measuring 1.4 metres across and about 5 centimetres thick. The segments will work together as a single huge mirror to collect tens of millions of times as much light as the human eye.

The segments must now be cooled, then their surfaces ground and polished to the right shape. If all goes right, they will make more than 900 segments (with about a 130 as spares), manufactured to have the telescope operational by 2024.


One comment

  • Localfluff

    Colossus is the idea of a telescope that does not cover a surface with lots of hexagonal mirrors. Instead it would consist of parabolic mirrors in a ring, or whatever formation, focused on a secondary mirror at or from where the image is reconstructed by magical interferometry. Unaligned mirror segments are diffraction limited, which somehow causes problems in optics that is bad for applications to study exoplanetary atmospheres. I suppose they need extreme precision spectroscopy to identify different molecules from the star light passing through an exoplanet atmosphere. And those problems are said to increase by the power of four with the number of segments, so bigger and bigger fragmented telescopes soon don’t contribute to the study exoplanetary atmospheres. Adaptive optics doesn’t fix this.

    Diffraction has to do with the wavelength of light. Very short for visible light. Without magical interferometry, mirror segments would have to be aligned at that nanometer level of precision. Separate parabolic telescopes converged by interferometry fixes that. I never understood much about optics. It was maybe Newton’s greatest contributions besides mechanics. Btw, Newton left 10 000 pages of writings about religion. He was an arianist (he concluded that the Father God and the Son God were different beings, whatever), which was punishable by burning to death in the witch hunting times he lived in. But he kept it secret and was burried as a hero of the nation. I have to take a look at what scholars have made out of that part of his work, which he spent most of his time with. Whatever Isaac Newton says deserves serious consideration.

    Interferometry is magic because it uses the fact that light is waves. Which is strange since I’ve seen light all my life. Even when I close my eyes, even when I sleep and dream, I see light. But I’ve never ever seen any waves in it. Even pure light hides something form us.

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