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SYFY WIRE Bad Astronomy

Gigantic Space Telescope’s Main Mirror Now Complete

By Phil Plait

Well, this is pretty cool news: The main mirror for the James Webb Space Telescope is now fully assembled!

OK, first, JWST is the successor* to Hubble, an observatory optimized for viewing the Universe in infrared wavelengths, outside what our human eyes can see. This will make JWST very sensitive to distant galaxies, low-mass stars, planets orbiting other stars, and about a zillion other very interesting astronomical objects.

And second, JWST’s mirror isn’t like other telescope’s, where you have a giant solid piece of glass. Instead, JWST’s mirror—which is 6.5 meters across!—is made up of an array of 18 hexagonal segments, each about 1.3 meters wide.

There are lots of advantages to this design; each mirror can be made much lighter weight than 1/18th of a big mirror, and mass matters when you’re launching a ‘scope into space. The mirrors are made of beryllium, which is very lightweight, so each segment has a mass of only 20 kilograms (45 pounds)!

Also, the entire assembly folds up like origami, allowing the completed mirror to fit inside the payload space of an Ariane 5 rocket. Finally, each mirror has its own independent actuators on the backside, allowing each segment to be individually adjusted to ensure perfect focus for the ‘scope.

The assembly of the main mirror is a big milestone for the observatory. It’s fantastically complex, and nothing quite like this has ever been flown into space before.

Oh, another thing about the mirrors: They’re coated with gold. Gold reflects infrared light very well (most glasses don’t), so it makes a great coating. Each mirror has a layer just a tenth of a micron thick; that’s 0.001 times as thick as a human hair! Even though it’s covering about 25 square meters in total, the layering is so thin that the total mass of gold used isn’t much, about 50 grams. The gold used is ultra pure and not cheap, but the kind of pure gold you can get on the market runs about $40/gram right now, so at that price JWST has only about two grand worth on it. That’s probably the least expensive part of the entire mission.

A lot of the work done on the mirror segments was performed at Ball Aerospace, just down the road from me in Boulder, Colorado. When the assembly was finished in 2012, they had a small press event, and I was able to attend. The highlight of that day was seeing one of the flight mirrors (that is, one of the actual segments that will fly into space as part of JWST’s main mirror) from just a couple of meters away! It was in a clean room, and I got a shot of it through a door:

Yes, that’s me reflected in the hexagonal mirror. That was a pretty cool day.

The final assembly of the mirror segments on to the "back plane" was accomplished this week at NASA's Goddard Space Flight Center in Maryland. The entire process took several weeks.

I’ve had varying opinions on JWST over the years; it will be a magnificent and ambitious space telescope, and will revolutionize infrared astronomy in much the same way Hubble did for visible (and ultraviolet) light. But it’s also had massive cost overruns and is far, far behind its original schedule, and that’s bruised NASA’s overall budget (and politics) for other astronomical missions over the years.

But while that still aches a bit for me, that doesn’t affect what this mission will hopefully accomplish: Give us the clearest, deepest, and best view of the Universe we’ve ever had at these wavelengths.

Congratulations to everyone involved in getting this important step done! And keep up the good work; there’s still a ways to go before the scheduled October 2018 launch.

*Over the years I’ve seen a lot of people refer to JWST as the “replacement” for Hubble. That’s just not correct; for one thing they look at different parts of the electromagnetic spectrum, so JWST can’t replace Hubble in that regard. Plus, if all goes well, Hubble will still be in use when JWST gets to work, so we’ll have both telescopes to peer into the Universe. One of the things I’m most excited about is having both of them look at some of the same objects at the same time; many phenomena are far easier to understand once you get different eyes looking at them.

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