The first mirrors for the European Southern Observatory's Very Large Telescope (ELT) arrived at Scope's future home in Chile in time for Christmas, but the unboxing process will take some time – and of course some assembly is required.
The 18 mirrors, which traveled around the world in late December, are the first of 798 hexagonal pieces that will eventually join together to form one. Very large 128 feet wide mirror. If all goes as planned, the giant mirror – the world's largest optical and infrared telescope – will begin collecting light from alien worlds, ancient galaxies and supermassive black holes in 2028.
This is what the ELT's primary glass sections look like before polishing at the Safran Reosc facility in France.
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Hundreds of hexagons
Even with the best modern glassworking facilities, it is still impractical to cast telescope mirrors wider than about 26 feet. Most of the world's largest telescopes have primary mirrors, which are actually assemblies of many smaller mirrors, carefully aligned and mounted in a frame. The ELT's glass sections are each about 4.5 feet wide and less than two inches thick. The surface of the glass must be perfectly smooth, and the margin for error is astronomically small: less than one-thousandth the width of a human hair.
„To achieve the required optical performance, the sections must be precisely calculated within fractions of a light wavelength. The average required surface quality is 15 nanometers,” says ELT's optical program manager Mark Carrel. Reverse. „The full width of a human hair is about 70,000 nanometers.”
The glass pieces traveled from a manufacturing facility in Germany, where the glass was cast, to another facility in France, where it was polished with a fine beam of ions, which, as ESO says, „scrapes the surface of the glass, removing irregularities atom by atom.” The technique, called ion beam imaging, has been used in several telescopes, including the Keck Observatory in Hawaii and the Gran Telescopio Canarias on the island of La Palma.
„The challenge is to industrialize existing techniques and processes so that the segments can be mass-produced (after a ramp-up of one per day),” says Carell.
Each glass is carefully packed in its own shipping container before being loaded onto a cargo ship for the long journey from France to Chile.
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Highly specialized shipping
Each batch of glasses, starting with the first 18, will have a long sea journey from France to Chile, then across the Atacama Desert to their home in the Cerro Amazons.
Forget bubble wrap and express mail: Glasses must be shipped in temperature-controlled containers equipped with special air cushions, and each piece must be placed in a special bag filled with dry nitrogen to protect the thin sheets of glass from warping due to humidity or moisture. Condensation is then installed on movement-stops in the container to protect against vibrations, bumps and other shocks.
„Even though we increase the security of the sections, ocean containers are transported to Chile using common commercial vessels and routes,” says Carrel. „The risk of damage is generally very low.”
Each glass section fits into a framework like this to support it and help it align properly with the other sections. Each of these assemblies weighs about 550 pounds.
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Gift boxed in polished silver
Now that they are in Chile, the glass parts must be carefully coated with an even thinner — 150 nanometer — layer of silver. The largest telescope, an array of four 8.2-meter telescopes on nearby Cerro Paranal, managed by the European Southern Observatory (which certainly has a consistent naming system), uses aluminum for the shiny metallic coating of its mirrors.
„By using a highly reflective silver coating, the ELT can collect more light – allowing ESO astronomers to observe the night sky in astonishing detail,” says Carell.
The silver coating adheres to the mirror's meticulously polished glass, thanks to a technique called magnetron sputtering – to put it very simply – applying a very thin, very smooth layer of silver to the surface of the glass in a vacuum. .
Once coated, the first 18 glass segments must then patiently wait for their 780 siblings to join them. Once they're all together, all 798 glass and silver hexagons are carefully aligned to form the ELT's primary mirror. That mirror captures light from distant objects and reflects it to a secondary mirror, which in turn reflects it back to instruments that help astronomers make sense of all those pixels.
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