ESO/F Carrasco
Extremely Large Telescope under construction in Chile's Atacama Desert
High in the mountains of Chile's extremely dry Atacama Desert, the European Southern Observatory (ESO) is currently building the world's largest optical telescope.
Choosing a name didn't take long: the telescope would be called the Extremely Large Telescope (ELT).
Instead, enormous energy is being poured into designing and building the “world's largest eye in the sky,” a satellite that is scheduled to start collecting images in 2028 and will very likely expand our understanding of the universe.
None of this would be possible without the most advanced mirror ever made.
Florent Mallet/Mersen Boustek
Dr. Elise Vernet inspecting the M5 mirror
Dr Elise Vernet is ESO's adaptive optics expert who has overseen the development of the five huge mirrors that collect light and direct it to the telescope's measurements.
Each of ELT's custom mirrors is a feat of optical design.
Dr. Vernet describes the 14-foot (4.25-meter) M2 convex mirror as a “work of art.”
However, the M1 and M4 mirrors perhaps best represent the level of complexity and precision required.
The primary mirror, M1, is the largest mirror ever built for an optical telescope.
“It has a diameter of 39 metres (128 feet) and is made up of 798 hexagonal mirror segments arranged to act as a perfect single mirror,” Dr Vernet said.
The M1 must be able to gather 100 million times more light than the human eye and maintain its position and shape with an accuracy 10,000 times that of a human hair.
M4 is the largest deforming mirror ever built, able to change shape 1,000 times a second to compensate for atmospheric turbulence and vibrations in the telescope itself that can distort images.
Its flexible surface is made up of six petals made of glass-ceramic material less than 2 mm (0.075 in.) thick.
The petals were made by Schott in Mainz, Germany, then sent to the engineering firm Safran Leosc outside Paris, where they were polished and assembled into a perfect mirror.
All five mirrors are nearing completion and will soon be shipped to Chile for installation.
These huge mirrors are used to capture light from space, but ESO's neighbouring Max Planck Institute for Quantum Optics in Garching has created a quantum mirror that operates on the tiniest scale imaginable.
In 2020, a team of researchers succeeded in making a single layer of 200 aligned atoms work collectively to reflect light, effectively creating a mirror too tiny to be seen by the naked eye.
In 2023, they succeeded in placing a single, microscopically controlled atom in the center of the array, creating a “quantum switch” that could control whether the atom was transparent or reflective.
“What theorists have predicted, and what we have observed experimentally, is that in these ordered structures, when you absorb a photon and it is re-emitted, it actually does so in one (predictable) direction, which acts as a mirror,” said Dr. Pascal Weckesser, a postdoctoral researcher at the Institute.
This ability to control the direction of light reflected by atoms could have future applications in a variety of quantum technologies, for example in hack-proof quantum networks for storing and transmitting information.
Zeiss
The world's most precise mirrors are manufactured by Zeiss of Germany.
Further northwest, at Oberkochen near Stuttgart, mirrors with other extreme properties are manufactured by Zeiss.
The optics company has spent years developing ultra-flat mirrors that are a key component in machines called extreme ultraviolet lithography machines (EUV), which print computer chips.
ASML of the Netherlands is one of the world's leading EUV manufacturers, and Zeiss mirrors are a key component.
Zeiss' EUV mirrors can reflect very short wavelengths of light, enabling sharp images at small scales and allowing many more transistors to be printed on the same area of a silicon wafer.
To explain how flat the mirror is, Dr. Frank Roemund, president of Zeiss Semiconductor Manufacturing Optics, uses a topographical analogy.
“If you scaled a domestic mirror to the size of Germany, it would have a maximum height of five metres. A space-based mirror (such as the James Webb Space Telescope) would be two centimetres (0.75 inches). An EUV mirror would be 0.1 millimetres,” he explains.
This extremely smooth mirror surface, combined with a system to control the mirror's position, also made by Zeiss, allows for a level of precision equivalent to bouncing light off an EUV mirror on the Earth's surface and finding a golf ball on the Moon.
While these mirrors may already sound extreme, Zeiss plans to improve them to help make even more powerful computer chips.
“We have ideas on how to develop EUV further. By 2030, the goal is to have microchips with a trillion transistors. Today, it's maybe 100 billion.”
This goal has been brought closer by Zeiss' latest technology, which can print roughly three times as many structures in the same area compared to current generation chip manufacturing machines.
“The semiconductor industry has a strong roadmap that inspires all players contributing to the solution, which allows us to bring about advances in terms of microchip manufacturing that today enable things like artificial intelligence that were unthinkable 10 years ago,” Dr. Lohmundo said.
It remains to be seen what humanity will know and be able to do in 10 years' time, but mirrors will undoubtedly be a central part of the technology that gets us there.
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