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SYFY WIRE Bad Astronomy

JWST’s Mirrors’ Golden Glow

By Phil Plait

That’s no flower. That’s the eye behind what will be the planet’s most amazing telescope.

The James Webb Space Telescope is currently under construction at NASA’s Goddard Space Flight Center just outside of Washington, D.C. The ‘scope is designed to observe the Universe in infrared light, and ordinary glass, even the substances used to make visible light telescopes, won’t do. Infrared light passes through it, or doesn’t reflect efficiently.

Gold, however, is an excellent reflector of IR light, so each of JWST’s 18 hexagonal mirrors is coated in an extremely thin layer of gold, only a fraction of a micron thick. Each mirror only needs about 3 grams of gold, which is actually pretty cheap compared with the other components of the observatory. It looks odd to our eyes, but to an object emitting IR light, those are nearly perfect mirrors.

This photo is pretty remarkable. The individual mirrors are about 1.3 meters (a little more than 4 feet) across, but weigh a mere 20 kilos (45 pounds). This was achieved by using lightweight beryllium for the backing, and cutting away most of that to leave a triangular ribbed web of metal. When you’re going to space, weight is an issue, but not at the cost of stability; beryllium gives both low mass and high structural support.

The gold is deposited on the mirrors using what’s called vacuum deposition; the mirrors are placed in a vacuum chamber and the gold is deposited as a vapor, allowing the surface to be built up at one atomic layer at a time. This allows for a very smooth finish, and high reflectivity for the mirror. The process is described in this “Behind the Webb” podcast from the Space Telescope Science Institute:

The mirrors were completed in 2012 at Ball Aerospace in Boulder, Colorado; I attended a celebration of the event. They’re pretty amazing to look at, beautiful in both color and symmetry. Incidentally, NASA has a webcam (called the WebbCam, of course) set up in the clean room where JWST is being prepped, so you can watch what’s going on live.

The final design has the 18 mirrors in the flower pattern you can see in the first photo. Lying across it is the V-shaped boom holding the secondary mirror (you can see it just in front of the bunny-suited technician standing on the orange platform on the left). After launch, that boom will extend up, so the secondary mirror is pointed down at the center of the primary array; light from celestial objects will bounce off the primary array into the secondary mirror, then back down through the central hole in the primary to the instruments (cameras) that will be behind the mirror. The hexagonal mirrors have actuators on the back that allow their positions to be adjusted; each will be carefully aimed so they act in concert.

Oh, did I mention the entire 6.5 meter–wide primary array will be folded up when it’s launched on the Ariane 5 rocket, then open up like the flower it resembles once it’s in space? Yeah, that’s pretty amazing, too. Once opened and adjusted, the mirrors should be aligned to almost nanometer accuracy.

That will happen about six days after launch, while JWST is still on its way to its final destination: a gravitational resting spot called the Langrange 2 point, about 1.6 million kilometers from Earth away from the Sun. There, the gravity of the two objects balances with the orbital centrifugal force, providing a volume of space where spacecraft can maintain position easily without too much adjusting.

At that point, the observatory is more or less on its own. We can’t send astronauts to upgrade or repair it like we have with Hubble, so all these intricate dances (including deploying the folded-up tennis-court-size sunshade) have to work pretty much perfectly.

This’ll be quite a thing when it all happens. Expect a lot of nervous tweets from me and every other astronomer on the planet when it does. But all this technology has been tested in one way or another on Earth and in space, so hopefully the procedure will go well, and the roughly $10 billion ‘scope will be ready to go. It’s bigger and in many ways more powerful than Hubble, able to detect planets around other stars, stars being born that are otherwise hidden by thick shrouds of dust, asteroids and comets in our solar system, galaxies at the thin hairy edge of the observable Universe … and who knows, maybe even Planet Nine, if it’s out there.

But that’s still in the future. But not too far in the future! 2018 is coming soon, and then perhaps we’ll see a revolution in astronomy as big as the one in 1990, when Hubble itself was lofted into space.

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