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They're made of beryllium, which is a really lightweight, stiff metal, instead of glass.
That's how we can make it really lightweight. Beryllium's got properties that we like when
it gets really cold. Everything changes shape when it gets cold, right? They shrink, typically.
So remember how I talked about one of the two engineering challenges, one being deployment?
The second one is this thing's really cold. We're building it at room temperature, a one-G,
one gravity environment on the surface of the Earth, right? But it's gotta work in a
vacuum, under weightless conditions and it's gotta work at extremely cold temperatures.
The whole part of the telescope on the shady part of the sun shade, that is going to be
about 400 degrees below zero Fahrenheit. So the whole cold part of the observatory is
going to be really cold, so we've actually had to build it exactly wrong so that it will
be exactly right at operating temperature. That's a real engineering challenge because
everything shrinks when it gets cold and they shrink at different rates and some things
stop shrinking at certain temperatures. Like beryllium basically stops changing shape below
100 Kelvin, which is one reason we like it. The hard part is it shrinks a lot from room
temperature to 100, but then after you get below 100 it stops changing shape. That's
really important for a mirror, right, because it's gotta have the right shape to focus all
of the light in the right place. Lots of different materials in this observatory and they change
at different rates, so that whole building it wrong so that it's working exactly right
at operating temperature is a big deal.