Building the biggest telescope dish ever made
To ensure that the deployment of the James Webb Space Telescope (JWST) is a success, NASA, with support from the European Space Agency (ESA) and the Canadian Space Agency, needs several crucial ingredients on board which can fold out in space, operate at extremely cold temperatures, and work to nanometre precision. Else it’s game over. One vital piece of kit is the mirror. Many telescopes rely on these reflective dishes to capture and focus light. The further you want to see, the bigger the mirror has to be, and the JWST has the biggest mirror ever made. Izzie Clarke spoke to Scott Willoughby, Vice-President of Northrop-Gruman, whose team are responsible for building it…
Scott - The mirror has got 18 hexagonal mirrors that then, in effect, form what’s called one primary mirror. There was a couple of reasons we did that and one of them is that the width of the mirror, the diameter of it, is 6½ metres.
Izzie - About the same height as four people standing on each others shoulders.
Scott - The top of the rocket we launch in is only 5 metres across so if we build one 6½ metre optic it wouldn’t work. The idea of building multiple segments allows us to put some of of those segments, three of them on one side and three on the other, and we put them on a hinge line. We actually fold them for the way it starts its life on the ground and we fold those along the side and then we can launch and we’re much narrower because then it fits in the side of that cone.
And part of the other reason just in short is, it’s really hard to build one 6½ metre optic. It’s also easier to manufacture 18 segments individually even though it’s hard to hold them together and align them. They’re then coated with a very thin level of gold because gold reflects infrared light. It looks pretty but we didn’t do it because it looks pretty.
Izzie - Was this always the intention to have this giant mirror that you can fold some of the parts or was that a design process that sort of came on as you were designing the project, and as you were looking at: how are we going to look at the very beginning of our universe?
Scott - Early on there was a lot of debate about how big of a mirror, but it was always known we wanted pretty much the biggest mirror we could build and then launch into space. If we could have conceived of building something bigger we probably would have but part of the trades that occur was what kind of material those mirrors were? That took a few years, did we want to build a series of glass mirrors - that’s what Hubble has, what’s called ultra low coefficient of glass? Or beryllium mirrors, and it turned out the beryllium won in the competition as the chosen thing for behaviours and what we thought we could produce. So there was more of a debate about how to build a big mirror, but people always wanted a big mirror.
Izzie - Beryllium is a relatively rare metal known for being strong but lightweight and, importantly, it doesn't distort at the sub-zero temperatures of space. But, getting it to the right shape wasn’t easy...
Scott - It took eight years to build the mirrors. There was a lot of planned iteration knowing this was hard. The one thing about building equipment that’s this advanced is you don’t tend to what I’ll say “go for broke” and say alright, here we go. We got 18 mirrors - now let’s go build em. You’ve got to polish them to what’s called a surface figure accuracy, how perfectly shaped is that surface? In our terms it’s 20 nanometres. I can only describe that as bacteria are on the order of nanometres. That means that the shape of this mirror is literally perfect almost to the size of atoms on the surface. But we knew it would be hard so at the beginning we built a development mirror. A little bit smaller but have the same materials and we polished it and proved we can make those features that perfect.
Then we started in production and to make 18 mirrors we couldn’t build them one at a time, we’d still be going. We literally had nine machines that were polishing so at any point in time we could have half the mirrors at a particular step or the other half were doing something else. It takes more than a village to build this many mirrors. Then you would polish it and make it more precise and then you’d put it back in the chamber, so we planned a very long cycle. We knew it would take a long time. The great thing was, at the end we ended up with 18 beautiful mirrors and a couple of spares that are just absolutely fantastic.
Izzie - What would be the damage if there was a small defect?
Scott - the part of the accuracy that drives us is we’re collecting photons (elements of light). We’re collecting photons that are 13.5 billion years old so these are the faintest photons so we’re not going to collect a lot of them. That’s why you need a bigger mirror so you can collect more. You have like a wider swimming pool, you’re going to collect more rain. So we’re collecting these photons and what happens is that the photons hit a primary mirror but they actually have to travel a course where they bounce. They need to bounce off the primary mirror out to a secondary mirror which is held by a tripod.
And then they bounce off a secondary mirror right down into the middle of the primary mirror where there’s another optical bench with more mirrors and instruments. Then they finally get to the place where detectors can collect these photons and that becomes our science data. You’ve got to capture the photon and bounce it into the right place. If the surface of the mirror is a little bit off, that photon won’t bounce to the right place and you’ll lose it. It'll just bounce right back out to space so you’ve got to make sure it bounces perfectly.