Webb Telescope's Mirror Size and Material
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NASA/ESA/CSA/STScI
The size of Webb's primary mirror—6.5 meters (21.6 feet) across—was determined by scientists and engineers in order for the telescope to observe distant galaxies at over 13 billion light-years away. The team also decided to make the mirror segments from beryllium, which is both strong and light.
Beryllium is a light metal (atomic symbol: Be) that is very strong for its weight and is good at holding its shape across a range of temperatures. Beryllium is also a good conductor of electricity and heat, and is not magnetic.