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Explained: The gold mirrors of the James Webb Space Telescope

The primary mirror is made of 18 hexagonal-shaped mirror segments — each 1.32 metre in diameter — stitched together in a honeycomb pattern.

webb telescopeWebb's primary mirror is 6.5 metres across; a mirror this large has never before been launched into space. (NASA)

The James Webb Space Telescope, NASA’s largest space science telescope ever constructed, is scheduled to be sent into orbit no earlier than December 22, 2021.

One of the most important objects it will carry is a large mirror which will help collect light from the objects being observed. The Webb Telescope team has built a mirror that can fold up and fit into a rocket and then unfold in space.

The primary mirror is made of 18 hexagonal-shaped mirror segments — each 1.32 metre in diameter — stitched together in a honeycomb pattern. When fully open it will be 6.5 metre in diameter. The Hubble Space Telescope’s mirror had a diameter of just 2.4 metre.

“The primary mirror is a technological marvel. The lightweight mirrors, coatings, actuators and mechanisms, electronics, and thermal blankets when fully deployed form a single precise mirror that is truly remarkable,” said Lee Feinberg, optical telescope element manager for Webb at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said in a release.

Each mirror segment weighs approximately 20 kilograms and is made from beryllium.

Why beryllium?

NASA explains that beryllium was used as it is both strong and light.

“Beryllium is very strong for its weight and is good at holding its shape across a range of temperatures. Beryllium is a good conductor of electricity and heat and is not magnetic. Because it is light and strong, beryllium is often used to build parts for supersonic airplanes and the Space Shuttle,” the agency said.

It added that special care was taken when working with beryllium because it is unhealthy to breathe in or swallow beryllium dust.

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So, it does not have gold?

It does. After the beryllium mirror segments were polished a thin coating of gold was applied to it. Gold helps improve the mirror’s reflection of infrared light.

The gold was coated using a technique called vacuum vapour deposition. The mirrors are kept inside a vacuum chamber and a small quantity of gold is vapourised and deposited on the mirror. The thickness of the gold is just 100 nanometers. So less than 50 grams of gold was used for the entire mirror.

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A thin layer of glass was also deposited on top of the gold layer to protect it from scratches.

First published on: 01-12-2021 at 16:51 IST
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