James Webb Space Telescope's primary mirror is ready to go!

By Phil Plait | August 20, 2012 6:30 am

The James Webb Space Telescope – NASA’s successor to Hubble – recently reached a pretty big milestone: all of the segments of its primary mirror have completed construction, and are ready to be handed over to NASA.

JWST isn’t your average ‘scope. Instead of a single, monolithic mirror, it will have 18 hexagonal segments that will fit together, working as a unit to focus infrared light from distant astronomical objects. Each segment is about 1.5 meters across, and will have actuators behind them (think of them as very accurately tunable pistons) to control exactly how the submirrors are aimed. On the front, each mirror is coated in a very thin layer of gold, which is an excellent reflector of IR light.

The mirrors were made at the Ball Aerospace facilities in my hometown of Boulder, Colorado. Ball threw a celebration to mark the mirrors’ completion, and invited a few press folks along. That included me! We went on a tour, and saw one of the mirrors – it was in a "clean room" to keep dust and other contaminants out. But we could see it through a door… and here it is:

Yes, that’s me reflected in one of JWST’s flight mirrors! That was pretty cool. [Click to embiggen.]

Looking a picture of a mirror can be difficult when you’re trying to see the mirror itself. Here’s another shot that makes it more obvious.

The mirrors is tilted up, and the dark band running through it is the reflection of the top of the stand it’s mounted in. Their mirror itself is the gold hexagon. I got a good look at it, and it’s no small thing for me to say its the cleanest mirror I’ve ever seen. I’ve been around a few ‘scopes in my time and their mirrors always have some schmutz on them. This had none.

The figure of each mirror (the technical term for the shape of the surface) is incredibly accurate: the bumps in the surface are on average smaller than 25 nanometers. A nanometer is a billionth of a meter… to give you an idea of how small this is, a typical human hair is 400 times thicker than the deformities in the mirror. As one person mentioned to me while we were gawking at the facilities, if a bacterium fell on the surface, it would far and away be the biggest thing on the mirror.

So yeah, these things are smooooth.

The mirrors need to be this smooth to accurately reflect light. Any nonconformity would scatter light a little bit, messing up the telescope’s resolution. I’ll add that mirrors like this – the size they are, made of beryllium, figured to this accuracy – have never been accomplished before. And that’s only part of it, since of course all 18 mirrors must act as one once JWST is in orbit.

We were shown a room with the tanks containing the completed mirrors, laid out on the floor in the same configuration they’ll be in the telescope itself:

As you can see, there’s an inner ring of hexagonal mirror segments and another ring outside those. Each individual mirror will focus light into another mirror, called the secondary, that will be mounted in front of them (I have pictures and video of a full-scale model of JWST that’ll help you see this). That will in turn reflect the light into a third mirror, which will then send the light from distant stars and galaxies into one of four main cameras.

And from there, we’ll learn a lot about how the Universe works.

To give you an idea of how big this assembly is, I’ve inset a diagram comparing JWST’s mirror size to Hubble’s. Hubble’s mirror is 2.4 meters across – about 8 feet. JWST’s mirror will be over 6 meters across – about 20 feet. It will be able to collect far more light than Hubble, and see fainter objects.

I’ve written quite a bit about JWST recent times. My opinions are conflicted (see here and here, and most recently here), but hopefully clear: I think this is a worthy project to do, but it has been plagued with cost overruns and is way behind schedule. It was originally supposed to launch in 2009 and cost less than a billion dollars; now it will launch no sooner than 2018 and the cost is estimated at $8 billion. This has put a huge amount of strain on NASA and other missions, both economically and politically. That money had to come from somewhere, so other missions have suffered to sustain JWST.

I’m hoping that most of that is behind us now. NASA is supporting the telescope, and Congress has given them enough money, hopefully, to complete it. I’ll note that Hubble itself had a history very similar to this: over budget, behind schedule, putting huge strains on relationships in the astronomical community. And now, of course, when people think of Hubble it’s with admiration – deservedly – for what it’s accomplished.

That may yet happen with JWST as well. Once it’s up and running, we’ll move on and see what a magnificent machine it is. But I also hope very much that NASA, Congress, and the astronomical community have learned the lesson of their big observatories. These missions need tighter and more realistic management if they are to be built within reasonable parameters, and not deleteriously affect other important missions.

It’s something… we should all reflect on.

Related Posts:

A model telescope (videoblog)
JWST shade in the made
The watershed moment for JWST
NASA’s budget: JWST saved, but not much good news


Comments (50)

  1. Cindy

    Hopefully after the Hughes Aerospace fiasco with the Hubble’s primary, that they’ve checked for spherical aberration and other grinding issues.

    In space, you don’t have to worry about dead moths falling on the primary, or dust settling on it.

  2. Robin

    Phil, I’m willing to bet the clean room in which these were coated is of a higher spec than the vacuum assembly used at observatories.

    To put some perspective on the surface figure (I’m assuming it’s 25nm RMS), the primaries on LBT are finished to 30nm RMS. There have been astronomy (and astronomy-like) projects completed with surface figure requirements as low as 1-7nm RMS. It’d be interesting to know how low the people finishing JWST’s mirror segments could have taken that surface figure. What a lot of folks don’t know is that it’s much harder to make a flat (like these mirror segments) than it is to make a spherical surface.

  3. Ian

    Phil, I thought this project got scrapped due to budget cuts!? This is supremely good news!

  4. Frank Harris

    Awesome! What an exciting landmark! However, I have a question.

    I’ve been an intern at GSFC for the past two summers, and both years I’ve gone to the High Bay Cleanroom to see the construction of the telescope. It seems that the mirrors are already there. What’s up with that? Are some in Colorado and some in Maryland? Or are the ones here just dummies for testing or something like that?

  5. SteveJ

    Come on Phil, admit it. Those are pictures of you in the Phantom Zone.

  6. Rob

    @Robin: These mirror segments aren’t flat, fwiw. I doubt that they’re spherical, either, though, probably paraboloid ( http://en.wikipedia.org/wiki/Three-mirror_anastigmat .)

  7. The cost of the JWST is about 1% of what the Pentagon spends in just one year to continue our military adventures overseas. Some very minor tightening of our defense budget could allow much more funding for valuable scientific research projects.

  8. davem

    The gold coating has obviously changed the reflected frequency of white light – does it also affect the infra-red?

  9. Robin

    @Chuck (#4):

    Make no mistake, JWST is hugely expensive. Leaving aside whether or not its continuation was a good thing, its cost and its continuation have hurt related projects at NASA and future budgets for similar projects. Hopefully the science that JWST will (another hopefully) return will have an impact that outweighs the negative impact the project has had. There are certainly projects that won’t fly as a result of JWST’s continuation.

  10. thetentman

    When is this scheduled to launch?

  11. Ross Cunniff

    @davem – good question. The short answer is “no” – see the chart in the Wikipedia entry on “reflectivity” for details http://en.wikipedia.org/wiki/Reflectivity – Gold is the Au line. Significantly less reflected in the visible light range of 400-700 nm (trending upward in the longer green and red wavelengths, causing the yellow color), but very close to 100% in the longer IR wavelengths.

  12. Robin

    @davem(#5): The reflectance of gold starts to get really good as the visible spectrum turns red and stays in the high 90’s (95%+) well out past 14μm. From about 0.9μm to 14μm, the reflectance is better than 98%. Gold sucks in the ultraviolet, as well as in the blue/green visible spectrum.

  13. Chris

    Phil, didn’t you see the sign? No cell phone photography! (bottom of second picture)

  14. John EB Good

    If no close-by robotic exploration of our solar system for a decade is the price to pay to see better and farther into this Universe’s past, I guess such a time traveling machine is better than no exploration at all and a kick in the b*tt!

    I’m sure, like for the Hubble, you’ll find ways to use it you haven’t thought of yet and things you weren’t looking for at all. And if this complex tool breaks, it may at least give one usefull mission to early Orions flights and their BAMF rockets that are the SLS. I guess it will the only ship able to ferry the Maytag Person, his/her toolbox and spare parts up there. If not, that could turn it into a really costly space junk at its weakest link.

    We will have, for the next few years after all, probes around or scouting by most planets of our solar system and they proved survivable way passed their manfacturer’s warranty period. It may be a well deserved budgetary break for this scope. But sending no further probes out there and not at the very least planning one or two just about now, stopping right after inventing the sky crane that could one day land supply containers and a modular base on Mars, isn’t an option if you guys want to continue to be taken seriously by the other space faring nations.

    When it’s time to leave Earth orbit, ask the Russians how they feel nowadays. Ask how proud they’ll be of their Soyuz when Dragon will replace it for good and they too buy their seats at SpaceX to access the ISS at way cheaper cost. That’s how Exploration turns into Colonisation: when there’s good money and good business to do out there!

  15. sebastian


    Yeah, I am also intersted in why Phil chose to harm the mirror and go against the rules…

  16. For those asking – this was a media tour, and they allowed us to take pictures. They did insist on checking each picture before letting us leave though. Making sure we weren’t revealing trade secrets, I guess. Or, also likely, Ball builds some spacecraft that are for intelligence work and don’t want too much detail revealed in pictures, since there’s probably tech overlap.

  17. kevbo

    I could write at least a couple of paragraphs saying my thoughts on Webb, but well, my thoughts would simply mirror yours…

  18. Brian

    It looks like that sign is banning flip-phone photography. I am sure Phil used his iPhone.

  19. MaDeR

    I have nothing against JWST per se. But I am very, very annoyed that other items was raided to fund JWST. It is just highway robbery and many things fell victim to it – from Mars exploration program (why do you think there is nothing, none, nada, nil planned after Maven? I will not even mention backing out of ExoMars) to astronomy.

    I really hope JWST sciencists are persona non grata at parties of sciencists belonging to other branches…

  20. Tony

    That mirror is going to be HUGE! It scares the heck out of me to think of how this telescope could be damaged simply by being rocketed into orbit. I have no doubt that all of the forces in play have been taken into consideration, but it’s still a scary thought.

  21. Astroblue

    You are so handsome and cool, Phil!

  22. Chris

    Will the images from the JWST be as aesthetically pleasing as Hubble’s images?

  23. Chris

    What is it about Gold that makes it such a good reflector of IR light?

  24. Robin

    That mirror is huge……for a space telescope. Consider how big it is compared to the average defect size on it: 6.5m vs. 25nm. If JWST’s primary was as wide as the US (3000 miles), the average defect would be a bit less than 0.75″ in size. How much the mirror segments deviate from the design right now is likely more than an average of 25nm, but each segment was figured so that when JWST reached station (at Sun-Earth L2) and ultimately cooled to the temp there…..well, then the surface figure would be no more than 25nm. So it’s gotta cool to its best possible shape.

  25. Robin

    @Chris (#18):

    The easy answer is that gold’s IR behavior is the result of it’s index of refraction. The deeper answer is that it’s the result of two particular electromagnetic properties of gold.

  26. Ted Hartley

    Is there a ‘7 Minutes of Terror’ equivalent movie for what must be the unfolding of the most expensive piece of origamy ever created? I will be holding my breath while the unfolding is happening, and I’m just a curious bystander.

  27. Artie

    I can’t believe you didn’t take any IR photos of the reflections in this mirror. That would have been hot.

  28. jph

    “Mirror, mirror in the hall, who’s the Baddest Astronomer of them all?”

    “Uh, Phil, looks like you got something between your teeth there.”

  29. Das Boese

    14. John EB Good:

    It’s unlikely you’ll ever see an Orion flight to fix the JWST. Leaving aside wether SLS will ever fly or be cancelled because it’s unaffordable like Constellation before it, the JWST isn’t designed to be serviceable. Just grappling onto it without breaking something would be hard, never mind having clumsy humans in bulky spacesuits poking around.

  30. Chris

    @18 Chris
    The other reason they chose gold is that it is easy to apply a thin layer of it and it does not tarnish easily.
    If you look at silver and aluminum
    you’ll see their reflectivities are very close to gold as well in the infrared part of the spectrum (on the right hand side). However, silver and aluminum oxidize very easily on Earth and mirrors made of these materials would need a coating which would severely impact the final mirror.

  31. Alan(UK)

    These very large, ambitious, projects are just what the US is good at (not just because they are the ones with the money).

    I am no fan of manned space travel. How much science can a man in a pressure-suit do? Who wants a stint of a few years at L2? Unmanned probes have visited, and will continue to visit, places in the Solar System that man can never realistically visit.

    It will be a shame when Hubble finally goes but it does work in one of the small parts of the spectrum that can be viewed from Earth. The future lies elsewhere – at wavelengths that penetrate all the muck out there.

    Having grown up when the latest and greatest was the 200inch Hale Telescope, with its fuzzy black-and-white images, I will be quite old when the JWST sees first light.

    I wondered why NASA did not call it the ‘Wernher von Braun Space Telescope’? It would have been fitting but perhaps the Europeans objected to having: ‘SS-Sturmbannführer Wernher Magnus Maximilian Freiherr von Braun’ on the side of an Ariane 5 (or maybe they liked the idea of a rocket with his name on – but it was going to the wrong place).

  32. Dutch Railroader

    @31 The “James Webb” ST is a perfect name. James Webb started space science at NASA, even though it was not part of his direct charter from JFK to figure out how to get to moon. Webb very deliberately moved to do more than Apollo, and the first space probes in the 60’s and all that followed were part of his legacy. I remember one of my mentors, now sadly long gone, who spoke of Webb calling up scientists, inviting them to come down to DC and help them figure out what to do.

    If JWST lives up to its promise, it will be a fitting tribute to Webb’s memory.

  33. Scott P.

    “It looks like that sign is banning flip-phone photography.”

    Maybe it is just saying no looking at photographs on your phone when you could be looking at the super cool telescope!

    Hard to say.

  34. Jess Tauber

    And yet I know how to build a gigantic telescope that weighs just a few ounces, with a mirror only a couple of atoms thick. Scale it up to whatever size suits you.

  35. Infinite123Lifer

    ok hold on. You got a picture of your FACE reflected in a lens from JWST? By 2020 Phil you could actually be the coolest person I don’t know.

  36. Gary Ansorge

    Very cool! Thanks, Phil.

    GAry 7

  37. That mirror is so clear it looks like a bad photoshop
    Or maybe TMA-1’s pimpin’ cousin

  38. Steve M

    Very cool Phil. Great story.

  39. Grand Lunar

    Awesome post, Phil!

    While it is a shame about how the JWST was mismanaged, I think that the potential return in science alone will make it worthwhile.

    I can hardly imagine just what we might find with such a large telescope!
    And since it “sees” in infrared, it should see through dust, right?
    With that being the case, I hope they turn it toward the Milky Way’s center to check out Sag A*.

    And might it also be useful for exoplanets?

    These are exciting times.

  40. Jim S.

    @ Cindy: The Hubble issue was the result of a piece of test equipment at Perkin-Elmer Corporation, not Hughes Aero.

    @ General, since resolution scales with wavelength, and because Webb is looking deep into the infrared, will the Webb simply be on par with Hubble in terms of resolution? In other words, in order to see deep in the past (highly red-shifted) I surmise they had to make the mirror huge just to have the same resolution as Hubble.

  41. Melf_Himself

    Phil, why are the movable segments needed? Is this to correct for the optics of the telescope itself? Is it because it’s too hard to make a mirror that large that is optically ‘perfectly’ shaped?

  42. Ken (a different Ken)

    #41 @Melf_Himself: Because a 20+ foot wide payload fairing makes for a frightfully expensive launch vehicle… The mirrors and sun shields all fold up into a nice little bundle for launch and transport.

  43. Daniel

    Are they sure this time? Its gonna be kind of hard to send up a corrective lens with no shuttle 😉

  44. Great news – can’t wait till first light for it! :-)

    @3. Ian : “Phil, I thought this project got scrapped due to budget cuts!? This is supremely good news.”

    If I remember right, the JWST certainly came pretty close to becoming a budget casualty. I’m glad it wasn’t but its going to be pretty nerve-wracking considering there’s only one James Webb great space robot observatory with just so much invested and so many hopes and possible science resting on a single launch going smoothly and successfully and being as designed once orbiting as (#43) Daniel has noted.

    I so wish there were at least two of these – preferably a whole production line of ’em – and we still had the spacecraft to visit and repair them if need be. The consequences if something
    were to go badly wrong I really hate to think about. :-(

    Then again, as with the Curiosity rover if we dare mighty things .. 8)

    @10. thetentman asked : “When is this scheduled to launch?”

    It’s Wikipedia page – linked to my name for this comment – says 2018.

  45. Wouter Lievens

    Space Mirrors of Catan, heh.

  46. Robin

    @Messier (#44):

    We’ve never had manned spacecraft that could reach Sun-Earth L2. L2 is about a million miles beyond Earth. There’s also the issue of getting men back from there and keeping them save from the radiation of deep space, capabilities none of our manned space craft have ever had.

  47. Messier Tidy Upper

    @ ^ Robin : Well, yes, that’s true and a good point.

    Although I guess if the Apollo spacecraft were able to go to the Moon they might’ve been able to get to L2 too, right? But then possibly not back again assuming they needed the Moon to whip them around and change their course so much rather than being able to decelerate to a stop, and turn around without it? Not sure.

    People were and still are talking about future human spacecraft visiting Mars and some asteroids in person so you’d think / hope we could find solutions to those problems.

    ‘Spose I should’ve said “if only we had developed the spacecraft ..” leaving out the “still.”

  48. Animal Mother

    2018 is a looong way off and JWST is going to be on the chopping block every fiscal year. It’s a HUGE money sink and I believe the chances of it actually launching are less than 50%.

  49. Matt B.

    Ball Aerospace? Neat, I’ve applied to a few jobs there. Never got a response, though.

    If anything goes wrong with the mirrors, I’d expect it to be that they switched some between the inner and outer rings or oriented them wrong. But I’m sure they check all that.


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