Date: March 25, 2010
Title: The Mirrors that Make James Webb
Podcaster: Roz Brown
Organization: Ball Aerospace & Technologies Corp. – www.ballaerospace.com
Description: Mention the James Webb Space Telescope and an almost endless list of superlatives follow. The biggest telescope to ever fly, once it’s launched in 2014. The first big telescope to operate at Lagrangian Point #2, a mathematical point in space roughly a million miles behind a full moon. The coldest big telescope to ever operate in deep space. And the first telescope with a fighting chance of seeing the first luminous objects to literally turn on shortly after the Big Bang. But if you want to see galaxies from 13 billion light-years away, you need a lightweight mirror that won’t weigh down the rocket carrying it into space. Roz Brown talks to Mark Bergeland, the Ball Aerospace program manager for JWST, about progress being made on the telescope’s amazing mirrors.
Bio: Ball Aerospace & Technologies Corp. supports critical missions of important national agencies such as the Department of Defense, NASA, NOAA and other U.S. government and commercial entities. The company develops and manufactures spacecraft, advanced instruments and sensors, components, data exploitation systems and RF solutions for strategic, tactical and scientific applications. Roz Brown is the Media Relations Manager for Ball Aerospace.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Jim West, wishing Happy 12th Birthday to my daughter Hannah who is a budding astronomer.
Roz: I’m Roz Brown, Media Relations Manager for Ball Aerospace. Today I’m with Mark Bergeland, Ball Aerospace’s program manager for the James Webb Space Telescope mission.
RB: Mark, a reporter with the Boston Globe has written that James Webb is a “21-foot diameter honeycomb mirror that will ride Aladdin-like atop a ‘carpet’ of tennis court-size sun shields” – a description I really like because the Aladdin character is a Disney creation and the James Webb when first proposed almost seemed so fantastic as to be something Disney thought up. But here it is, four years away from launching. You are overseeing Ball Aerospace’s role for the James Webb mirror. What makes this mirror different than other telescope mirrors?
MB: First there’s just its size. It’s a six-and-a-half meter diameter – roughly 21-foot-diamter mirror and that’s much larger than anything that’s flown in space before. And also it’s not just “a” mirror, but 18 individual mirrors that are all working together in space to look like one big mirror. The mirror itself is too big to fit into any launch vehicle and so it has to fold up origami-style to fit into the launch vehicle and then once on orbit it deploys out into this large, 21-foot mirror.
RB: There are several other things that make James Webb very distinct, Mark. What are those?
MB: Well, the primary mirrors are made out of beryllium, which is a very light-weight metal. Early on in the program we did a trade as whether to use a traditional glass mirror, or to use beryllium metal mirrors. And it turns out that for the cryogenic application – the very low temperature that we’ll be operating at – beryllium is a better choice. The telescope is cryogenic, which means it operates at roughly minus 400 degrees Fahrenheit. And that’s important because it’s an infrared telescope that will allow us to see the very faint, infrared objects that are visible from the very beginning of the galaxy – formation of galaxies and stars – just shortly after the big bang.
RB: So, beryllium was chosen to create the telescope’s 21-foot diameter mirror. Not many of us have any reason to ever know much about beryllium, so why beryllium?
MB: Well, beryllium is a naturally occurring element. It’s number four on the periodic table. It’s a very light-weight, very stiff material that’s ideal for aerospace applications. In addition to the telescope being very large, it also has to be very lightweight in order to be able to launch it on a launch vehicle, and beryllium is an excellent choice for that. Each of the individual mirror segments – the 18 hexagonal segments that make up the primary mirror, start out weighing about 540 pounds and then we go through a machining process and the beryllium material allows us to machine most of that mass away and what’s left is a very thin ribbed structure that weighs only about 45 pounds per mirror.
RB: So really, it will be the largest and the most light-weight mirror ever launched?
MB: Yes. The primary mirror that we’re building is six-and-half meters in diameter compared to Hubble’s primary which was about two-and-a-half meters, and yet the entire observatory for James Webb weighs about half what the Hubble Space Telescope weighed.
RB: So, that’s just how much technology has advanced.
MB: Yes. It’s very advanced and unfortunately it’s very challenging so we’re pushing the state of the art every day as we learn how to build these mirrors to the fine optical performance requirements that we have for James Webb.
RB: So James Webb is using beryllium, it has a hexagonal shape and it’s cryogenic. And
there’s also the challenge of aligning it on orbit. Tell us about that.
MB: Well, each of these individual mirror segments, when we launch the spacecraft, will go through a vibration and kind of a rough ride getting onto orbit and we expect that they’ll get misaligned a little bit. And one of the things we’ll need to do to commission the telescope is to align each of those 18 individual segments that make up the primary mirror into one large primary mirror. So each segment has its own actuation structures and motors that allow us to drive it to the position it needs to be so that it looks like one large mirror rather than 18 individual segments.
RB: Has a telescope ever been aligned on orbit?
MB: Not to this extent. Telescopes that have flown in the past typically have a single, primary mirror and they’ll have a focus mechanism or something like that, that will allow the telescope to get into fine focus, but James Webb essentially has to be constructed in space, and nothing like that has every been accomplished before.
RB: So how does that work, exactly as far as actuators on each individual mirror?
MB: Well, there are actuators that first take the folded-up observatory and deploy out it into its rough shape, but then each individual mirror segment has actuators that do the fine positioning, moving in six degrees of freedom: tipping, tilting, clocking – all those motions that are necessary to get it right in the exact, correct position. And that’s accomplished by small actuators that are mounted on the back of each mirror. Those actuators initially travel about a half inch to get the mirrors off the launch locks that hold them in place during the launch, but once they’ve moved that coarse move, they go into a fine positioning mode where they move on the order of 10 nanometers at a time. A nanometer is a billionth of a meter – so very small motions that are required to align the telescope segments to just a fraction of a wave length of light.
RB: All of this heavy lifting, so to speak, being done by the actuators – this isn’t visible by the human eye, right?
MB: No, the motions that the actuators make are very small – on the order of 10 to 12 nanometers when we’re doing the final phasing of the telescope. Just as a point of reference, a sheet of paper is about a 100 thousand nanometers thick, so you can get a sense for how small these motions are. The final phasing gets the telescope deployed to within a 130 nanometers of an ideal optical surface and those actuators are doing the final tweaking in getting those mirrors just where they need to be.
RB: And all of this had to be tested in advance before work could even begin on the actual telescope, on a small mirror or prototype, so can you tell us about that?
MB: Yes. Two of the real concerns when we started this program, for the mirrors at least, were could they survive the launch environment and not change shape due to the vibration and also, once we get on orbit, can we really align them to make those 18 individual segments look like one big mirror. So, we went through a series of tests to demonstrate those two technologies. The first was to build a full-size mirror and run it through optical testing to measure its shape, and then simulate the launch environment – the vibration environment and then measure the optical shape afterwards and confirm that it didn’t change during that launch environment. The other technology that we needed to demonstrate was the ability to align those 18 mirrors on orbit to look like one big mirror. And to do that we built a one-sixth scale test-bed telescope. It has the 18 individual segments just like James Webb will, but they were much smaller. But they did have the actuators on the back that enabled us to demonstrate that we could take the images from the telescope, analyze them, and determine how the actuators should be moved – to move those individual segments into position to look like one large mirror.
RB: Where are we in the process of completing work on the James Webb Space Telescope mirrors?
MB: Well, the mirrors take a long time to produce. We actually started the initial development of the mirrors, almost seven years ago. The program recognized that the mirrors would be very difficult, so we started with the early development of the materials, and the designs to build those mirrors back in the 2003 timeframe, and we immediately started off into the actual fabrication of the mirror segments because each segment takes about five years from start to finish. Right now we have all of the 18 mirrors in their final polishing stages at a subcontractor called Tinsley out in the Richmond, California area.
RB: And it’s a real globe-trotting process for these mirrors, right?
MB: Yes it is. People all over the country are working on James Webb. The mirrors start as raw beryllium blanks, then get formed and are then machined at another supplier. They move to Colorado for assembly and then to the West Coast for polishing and then they go down to the Marshall Space Flight Center in Huntsville, Alabama where we do the initial cryogenic measurements of the mirrors. That’s an important step – because we know when the mirrors go cold, down to minus 400 degrees, they will change shape – everything as it goes cold changes shape. The nice thing about beryllium is that it changes shape repeatably. So, the first time we take them to Marshall Space Flight Center and simulate the space environment we go to the minus 400 degree temperature that the telescope will operate at and the mirrors deform. And we measure that deformation and then go back to the polishing house and essentially polish the inverse of that motion into the shape of the mirrors so that the next time they go cold, when they go on orbit, all the ridges and highs and lows and valleys on the mirrors will level themselves out because we’ve polished the inverse into the mirrors.
RB: What’s the most fun you have being the program manager for this?
MB: I think the most fun is just working with all the talented people around the country who really have given their heart and soul to making this project happen. Everything from the astronomers who identify what they want to see and the engineers who put that into practice and build the hardware that makes that happen. We’re about eight years into the program and that’s gone like the blink of an eye and yet the remaining few years to launch I’m sure will go very slowly because we’re all anticipating that day.
RB: Thank you Mark.
MB: Thank you Roz.
End of podcast:
365 Days of Astronomy
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