The mirror of the James Webb Space Telescope: looking into the past

by Haygen Warren

When the joint NASA/ESA/CSA James Webb Space Telescope launches atop an Ariane 5 rocket this December, it will carry with it one of the largest telescopic mirrors ever developed. Considerably larger than the Hubble Space Telescope’s 2.4 m mirror, James Webb’s mirror will be a massive 6.5 diameter mirror, made up of 18 hexagonal, gold-plated beryllium mirror segments.

Furthermore, the mirror of James Webb will be one of the most complex spacecraft systems ever launched, and — to no surprise — it takes an expert team, lots of development, and a plethora of testing to prepare the mirror of James Webb for launch and operation at Lagrange Point 2 (L2).

To gain insight into this incredible mirror and how it will function, NASASpaceflight interviewed NASA’s Lee Feinberg, James Webb’s Optical Telescope Element Manager.

When discussing any space telescope mirror, many will think back to the infamous defect in Hubble’s mirror in the 1990s — when the telescope’s mirror had been polished the wrong way, causing severe spherical aberration. Thankfully, Hubble was able to be serviced by Space Shuttle missions, so new, corrective instruments and fixes were launched and placed in the telescope.

However, James Webb can’t be serviced by a spacecraft due to its orbit. What’s more, James Webb, unlike Hubble, has 18 smaller mirrors that combine to make the telescope’s main mirror.

So what were the detailed sets of tests done to ensure the mirror has no defect, and what steps were taken to make sure that any single mirror segment does not have any aberration?

Hubble seen through the Flight Deck windows of the Shuttle Atlantis on STS-125. (Credit: NASA)

“So the lessons learned [from Hubble] were put into a report that was done on Hubble. It was led by Sky Luallen, a former director of the Jet Propulsion Lab, but it was called the Allen report and they had a series of recommendations of things we needed to do to avoid the Hubble kind of problem. And it turned out we were doing all those things already,” said Feinberg.

“The thing that really plagued Hubble isn’t that they didn’t take an independent measurement. They actually measured the primary mirror with two different null lenses, but the problem was they discounted the data from the one set of one lens that they thought was less precise.”

“In our case on Webb, what we did is establish criteria for each test we do beforehand, and we always make sure we meet those criteria. We’ve also been very transparent about sharing that data. We have an independent committee, some of whom had lineages back to Hubble, that looked at our data. So that was the second thing. Then we did a series of cross-checks along the way.”

This ensures a Hubble-like problem won’t occur with James Webb’s mirror, which is made up of 18 smaller, gold-plated beryllium mirrors that combine to create the iconic hexagonal mirror of the telescope.

Beryllium is very lightweight, hence why the James Webb mirror team chose it when designing the mirrors.

“It is true that beryllium is very light and has what we call a very high stiffness-to-mass ratio, meaning not only is it light, but it’s very stiff for the amount of mass that it weighs. You would think that’s the number one reason why we chose beryllium, but that actually wasn’t the primary driver, although it is a side benefit,” Feinberg said.

The James Webb Space Telescope in final integration at Northrop Grumman’s Redondo Beach, CA, facility. (Credit: NASA/Chris Gunn)

“You know, one of the real grand challenges of building Webb was figuring out how to make it lighter. Its mirrors are 10 times lighter per unit area than we used on Hubble but they operate at the cryogenic temperatures of the Spitzer mirrors. Ultimately, we were gonna have to make segments of it [the mirror] and we’re going to have to mass manufacture it.”

“So, therefore, it also had to be very lightweight, but also very precise.”

After running several design programs with the Department of Defense while the telescope was in development, Feinberg and his team found that beryllium was the best element to use for the mirrors given the conditions.

“And it turned out that beryllium’s advantage was actually its performance. But it wasn’t so much the mass, it was two things — its coefficient of thermal expansion at cold temperatures. The reason why you want a mirror that’s stable at your operating temperature is if there are small deviations of temperature, your mirror won’t change its shape,” Feinberg explained.

Unlike ground-based telescopes though, the team can’t update or move the mirrors of Webb until two weeks after launch.

“Webb has to be stable for two weeks on end before we update the mirror positions, so the mirrors themselves and the backplane all have to be stable. So that coefficient of thermal expansion at our operating temperature, which, for the mirrors, varies between 30 and 55 Kelvin… Beryllium is this amazing material at those temperatures. If you look at a coefficient of thermal expansion curve, you’ll see how flat it is, which means even if there’s a temperature change, nothing moves.”

“The second thing that’s beautiful about beryllium is that it’s thermally conductive. It acts like a metallic in terms of its thermal conductance, and what that does is make the mirror a uniform temperature.”

James Webb is an infrared telescope, unlike Hubble which is a visible light telescope. To maximize the amount of infrared light observed by the telescope, the mirror teams covered each mirror in gold — an element that is good at reflecting infrared light.

“You know how reflection works from an electromagnetic’s POV because, really, different materials reflect differently because of their molecular and atomic structure, how they form in terms of whether something’s a crystal or metallic, and then how the electrons are formed and how they reflect light. It turns out that gold, when you look at its properties, has the properties of very high reflectance.”

Furthermore, to enhance the amount of light the telescope can observe, the James Webb mirror team coated a protective substance on the gold mirrors that allows the telescope to observe a large range of wavelengths.

“I should add that we do overcoat the gold with a very protective surface so it’s well protected. There are more details of how people build coatings to be optimal, but one of the things that’s unique about Webb that you probably haven’t thought about is if you really look at the active wavelength range of Hubble, it goes from roughly Lyman-alpha, which is around 121 nanometers, up to about two and a half microns, and that’s using an aluminum coating,” said Feinberg

“Compare that against Webb, that goes all the way up to 28 microns, and then we really have sensitivity into the visible [wavelength] at about 0.6 microns. So we’re covering over 27 microns of wavelength range. That’s a huge wavelength swath with incredible reflectivity. That’s better than aluminum in the visible.”

Mirror segments are tested for the James Webb Space Telescope. (Credit: NASA/C. Gunn)

“Gold is an amazing material in how well it reflects light over such a large wavelength range. So we’ve really benefited from two gifts of nature in order to get the most out of Webb.”

Each individual mirror on Webb features six-fold symmetry, or hexagonal shapes that can be rotated by either 60° or 120° without changing their appearance. So, why is it important for James Webb to feature six-fold symmetry and not previous space telescopes?

“For Webb, the reason why we went with a hexagonal shape is that it’s really about efficiency in terms of needing to unfold the mirror,” Feinberg said.

“Now, how did we know that? Well, in order to do the science you start with why is Webb so big. Well, the scientists realized that we needed a mirror of a certain size really for two reasons. One is the larger the mirror the better. If you work at a longer wavelength where you go from visible to infrared, you go up by a factor of four or five if you want to have an equal resolution to Hubble.”

“You need to have a mirror that’s about four or five times bigger, and so that’s one of the things we really wanted. The astronomers said ‘We really need that level of resolution.’ The resolution they have on Hubble they use to look at the early universe and the Hubble deep field, but they also want to be able to have good sensitivity in the infrared because they need to look in the infrared to see the red-shifted light from the early universe.”

“So you have a bigger aperture for resolution, but then you also need sensitivity. You need a collecting area. And all of that led them to need to say we need a mirror that’s got about 25 square meters or more collecting area, which is a little less than what Webb is now,” Feinberg said.

In the weeks following launch, James Webb will coast through space toward its final destination — Lagrange Point 2 (L2). During the nearly month-long trek to L2, James Webb will deploy all of its instruments in preparation for its first infrared science observations.

“The mirrors actually don’t deploy immediately as you probably know. The wings and the secondary mirror unfold, but they both are part of the first two weeks of deployments,” Feinberg said.

“We have a whole special team that all they worry about is deployments, and so the deployment team is worrying about the deployment aspects of things. But from a mirror point of view, the kinds of things that we care about we’ll be paying special attention to at that point is the temperatures of the mirrors, especially early on during the very early phase of the mission, and then as they’re cooling down just to make sure things work right from a temperature point of view.”

“The other thing we pay a lot of attention to is, as things cool down, you have to make sure that you don’t form ice on any key surfaces. Water that can outgas from things that are warm, like a spacecraft, you need to keep off of the cold surfaces, especially as they get below a certain temperature where water will condense.”

“But that isn’t until day 15 or so. About two weeks into the mission we will finally start deploying the actual mirror segments themselves. The primary mirror segments and the secondary mirror deployment is over a week and a half-ish type process because the mirrors, when we launch, are in a stowed position and we will have to deploy each one of them roughly a half of an inch. We go in small increments and we carefully monitor each mirror. We have to make sure the mirrors can get close to each other, so there’s a lot of details.”

“The other thing that’s critical is making sure that after the wings and the secondary mirror support structure are deployed, they get latched into place. And we will want to pay close attention and make sure that all the latching is good. That’s the kind of telemetry that we will pay special attention to from the telescope perspective to make sure that we’ll be ready to start deploying mirrors.”

However, as James Webb undergoes its rigorous deployment process, backup systems are essential in the off-chance something goes wrong during mirror deployment. So how much redundancy is built into the telescope deployment systems?

“All of the electrical systems, all the wiring, even the windings on motors, all those kinds of things are redundant. But on the mechanical side of things, we usually call those things non-credible single-point failures. They are places where if something didn’t deploy, it would be problematic,” Feinberg explained.

If worse comes to worst, however, would James Webb still be able to operate if a major mirror system, such as the two foldable wings of the primary mirror, didn’t deploy?

“The only thing I can tell you is that we certainly cannot meet our baseline requirements. Without the wings, we would not meet the requirement for [mirror] area. Where we still could meet a minimal set of requirements is if one mirror does not fully deploy. And we do have contingency plans for what to do if we had an issue, for example, with one of the mirrors.”

“For example, we could point the mirror out of the way if we have some actuators working, or we could defocus it. So we could potentially get by without one of the mirrors. I think it’s extremely unlikely that that would happen, though, to be very honest with you. The mirrors have been really well tested, and there’s all the redundancy you would want.”

“The big deployments are different because they are more difficult to test in a 1-g environment. For example, we deployed mirrors during a test at temperature and we deployed them at different gravity orientations. So we’ve really covered all the potential environments. So even though we could live without a mirror, it’s just incredibly unlikely.”

Protected from the Sun’s rays by its multi-layer sunshield, the James Webb Space Telescope opens its mirrors to observe the universe. (Credit: Nathan Koga for L2/NSF)

After making its month-long trek to L2, what will be the first light, other than from itself, that James Webb will see?

“So first of all, the very first light that the observatory sees comes from the main science camera NIRCam. So we have to wait for NIRCam to get cold enough so that its detectors are cold enough. It turns out that with infrared detectors, especially the types that we use for astronomy, when they’re warm, they produce so much background noise that you wouldn’t even see an image,” Feinberg explained.

The first deep-space object James Webb will likely observe will be a bright star, and the team will use that star to assist James Webb with mirror alignment. The specific star will likely be determined from a deep-field type image.

“I mean, we know it’ll be a bright star that’s isolated,” Feinberg said.

“We will get some first light with NIRCam somewhere after the first month. That’s when I get really excited because then we’re starting to see things getting cold enough to where we can actually do something. The first thing we do is we take NIRCam and we create a mosaic. We’ll make a mosaic of NIRCam images and look at a bigger field of stars.”

“We’ve developed algorithms for determining where we’re pointing and then we can finely tune where we’re pointing and start the aligning sequence that will take over three months.”

However, the mirror team has not yet determined what specific star they’ll use for alignment. “We haven’t chosen that yet, and that will depend exactly on when we launch, too. They’ll wait till we’re a few weeks away from launch before they choose the exact star.”

(Lead image: James Webb Space Telescope after completing its comprehensive systems test in March 2020. Credit: NASA/Chris Gunn)

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