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JWST Observatory: A New Type of Mirror, a New Type of Telescope

The James Webb Space Telescope (JWST) will visit a time when galaxies were young. To do this, JWST will see galaxies billions of light-years away, further away than any other telescope has observed.

The optics (mirrors and lenses) are the heart of any telescope. To complete JWST's mission, NASA needs to build a new type of mirror to be the main mirror for a new type of telescope. Follow along to read about building the James Webb Space Telescope's mirror.

The Challenge: A large, lightweight mirror

JWST needs a large mirror to collect as much light as possible to see galaxies from 13 billion light-years away. The JWST scientists and engineers have determined that a primary mirror 6.5 meters (21 feet 4 inches) across is needed to measure the light from these galaxies. Building a mirror this large is a challenge, even for use on the ground. A mirror this large has never been launched into outer space before.

No rocket is large enough to hold a 6.5-meter mirror if it was all one piece. The JWST team decided to build the mirror in several segments, which will fold up to fit into the rocket and then unfold after launch.

JWST mirror assembly segment

JWST and Hubble mirror comparison
In addition to making the mirror small enough to fit into a rocket, the JWST team also has to make it light enough to be launched. If the Hubble Space Telescope's (2.4 meters) were scaled to be large enough for JWST, it would be too heavy to launch into orbit. The JWST team had to find new ways to build the mirror so that it would be light enough---only one-tenth of the mass of Hubble's mirror per unit area. Despite the fact that the mirror will be very light, it needs to be very strong to hold its shape.

A Cold Mirror

To see the first stars and galaxies in the early Universe, astronomers have to observe infrared light, and use a telescope and instruments optimized for this light. Infrared is similar to light, but human eyes can not see it. However, warm objects glow with infrared light. If JWST's mirror was the same temperature as the Hubble Space Telescope's mirror, the infrared light from distant galaxies would be lost in the infrared glow. Thus, JWST will need to be very cold ("cryogenic"), with its mirrors around -220 degrees C (-364 degree F). The mirror must be able to withstand very cold temperatures and hold its shape.

To keep JWST cold, it will be sent into deep space, far from the Earth and the mirrors will be shaded from the Sun's heat.

Meeting the Challenge

As JWST needs a new kind of mirror to meet these requirements, NASA set out to research new ways to build mirrors for telescopes. The Advanced Mirror System Demonstrator (AMSD) program was a four-year partnership between NASA, the National Reconnaissance Office and the US Air Force to study ways to build lightweight mirrors. Based on the ASMD studies, two test mirrors were built and fully tested. One was made from beryllium by Ball Aerospace; the other was built by Kodak (now ITT) and was made from a special type of glass.

A team of experts was chosen to test both of these mirrors, to determine how well they work, how much they cost and how easy (or difficult) it would be to build a full-size, 6.5-meter mirror. The experts recommended that beryllium mirror be selected for the James Webb Space Telescope, for several reasons, such as because beryllium holds its shape at cryogenic temperatures. Based on the experts team's recommendation, Northrop Grumman Space Technology (the company that is leading the effort to build JWST) selected a beryllium mirror, and the project management at NASA's Goddard Space Flight Center approved this decision.

What is Beryllium?

a marble sized piece of beryllium
Beryllium is a light metal (atomic symbol: Be) that has many features that make it desirable for the JWST primary mirror. In particular, beryllium is very strong for its weight and is good at holding its shape across a range of temperatures. Beryllium is a good conductor of electricity and heat, and is not magnetic. (At left is a picture of a marble-sized piece of Beryllium)

Because it is light and strong, beryllium is often used to build parts for supersonic (faster-than-the-speed-of-sound) airplanes and the Space Shuttle. It is also used in more down-to-Earth applications like springs and tools. Special care has to be taken when working with beryllium, because it is unhealthy to breathe in or swallow beryllium dust.

How and Where the Beryllium Mirror is Made

The beryllium to make JWST's mirror is be mined in Utah and purified at Brush Wellman in Ohio. The particular type of beryllium used in the JWST mirrors is called O-30 and is a fine powder. The powder is then placed into a stainless steel canister and pressed into a flat shape. The steel canister is then removed and the resulting chunk of beryllium is cut in half to make two mirror blanks about 1.3 meters (4 feet) across. Each mirror blank will be used to make one mirror segment; the full JWST mirror will be made from 18 hexagonal (six-sided) segments.

Beryillium Mirror Segments, Axsys Technologies
Beryillium Mirror Segments, Axsys Technologies

Once the mirror blanks pass inspection, they are sent to Axsys Technologies in Cullman, Alabama. The first two mirror blanks were completed in March 2004.

Axsys Technologies then shapes the mirror blanks into their final shape. Because it takes a long time to work with beryllium safely, Axsys is building a new facility that can process 8 mirror blanks at the same time.

The process of shaping the mirror starts with cutting away most of the back side of the beryllium mirror blank, leaving just a thin "rib" structure. The ribs are only about 1 millimeter (about 1/25 of an inch) thick. Although most of the metal is gone, the ribs are enough to keep the segment's shape steady. The front surface is then smoothed out and shaped properly so that it will be ready for its final position in the large mirror. Once Axsys has shaped the mirror segment, it will be sent to Richmond, CA, where SSG/Tinsley polishes it.

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Watch a movie about how the mirrors are made!

SSG/Tinsley starts by grinding down the surface of the mirror until it is close to its final shape. Then, the support structure for the mirror segment is mounted on the back, including small mechanisms to help focus the final mirror. After that is done, the mirror segment is carefully smoothed out and polished. Then the Tinsley team will then use a laser and an interferometer to analyze the mirror for accuracy and quality. They repeat this many times until the whole mirror segment is perfect. Once the mirror segment is polished, it travels to NASA's Marshall Space Flight Center in Huntsville, Alabama.

Since many materials change shape when they change temperature, a team from Ball Aerospace uses a cryogenic test chamber at NASA's Marshall Space Flight Center to test the shape of the mirror segment when it is cold. The mirror segment is cooled down to the temperature JWST will expericence in deep space, -220 degree Celsius (-364 degree F) and the shape of the mirror surface is checked again, using a laser and interferometer like before. If any problems are identified, the mirror segment travels back to California, where the Tinsley team reworks the mirror segment accordingly.

Once the mirror segment's shape is correct at cold temperatures, a thin coating of gold is applied. Gold improves the mirror's reflection of infrared light. Once the gold coating is applied, the mirror once again travels back to NASA's Marshall Space Flight Center for one final test of its shape. Once the mirror segment is complete, it travels to NASA's Goddard Space Flight Center in Greenbelt, Maryland.

ITT (formerly Kodak) combines the 18 segments into one big mirror in a special facility at NASA's Goddard Space Flight Center. In addition to the mirror segments, a mirror backing structure, built by ATK in their facility in Salt Lake City, Utah, is sent to Goddard. ITT mounts the mirror segments onto their proper place on the backing structure. The backing structure holds 12 segments in the middle part of the mirror, and has two wings with 3 segments each. The wings fold back so that the full mirror will fit into a rocket.

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Once the full mirror is built, it will become a key part of the James Webb Space Telescope. Once the telescope is built and the scientific instruments are added, the completed observatory will go through another round of testing to make sure that the JWST is ready to survive theheat, vibration and shock of riding into space on a rocket and the cold and vacuum of outer space.

Once the JWST team agrees that the James Webb Space Telescope is fully operational and ready to go, the mirror and the rest of the observatory will take one more trip. Blasting off on top of a rocket, JWST will take three months to reach its orbit at the L2 region of space , about 1.5 million kilometers (around 1 million miles) from the Earth.

Learn More View our mirror image gallery and our mirror video archive.
Learn More We have a piece on JWST's mirrors in our technology section here.

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