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The Primary Mirror

An Overview

One of the James Webb Space Telescope's science goals is to look back through time to when galaxies were young. Webb will do this by observing galaxies that are very distant, at over 13 billion light years away from us. To see such far-off and faint objects, Webb needs a large mirror. A telescope’s sensitivity, or how much detail it can see, is directly related to the size of the mirror area that collects light from the objects being observed. A larger area collects more light, just like a larger bucket collects more water in a rain shower than a small one.

Webb Telescope's scientists and engineers determined that a primary mirror 6.5 meters (21 feet 4 inches) across is what was needed to measure the light from these distant galaxies. Building a mirror this large is challenging, even for use on the ground. A mirror this large has never before been launched into space!

JWST and Hubble mirror comparison

If the Hubble Space Telescope's 2.4 meter mirror were scaled to be large enough for Webb, it would be too heavy to launch into orbit. The Webb 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 - yet very strong.

The Webb Telescope team decided to make the mirror segments from beryllium, which is both strong and light. Each segment weighs approximately 20 kilograms (46 pounds).

The Webb Telescope team also decided to build the mirror in segments on a structure which will fold up, like the leaves of a drop-leaf table, so that it can fit into a rocket. The mirror would then unfold after launch. Each of the 18 hexagonal-shaped mirror segments is 1.32 meters (4.3 feet) in diameter, flat to flat. (Webb's secondary mirror is 0.74 meters in diameter.)

JWST mirror assembly segment
The diagram above shows the three
different mirror prescriptions that the segments have.

The hexagonal shape allows a segmented mirror with high filling factor and six-fold symmetry. High filling factor means the segments fit together without gaps. If the segments were circular, there would be gaps between them. Symmetry is good because there need only be 3 different optical prescriptions for 18 segments, 6 of each (see above right diagram). Finally, a roughly circular overall mirror shape is desired because that focuses the light into the most compact region on the detectors. A oval mirror, for example, would give images that are elongated in one direction. A square mirror would send a lot of the light out of the central region.

JWST mirrors

Once in space, getting these mirrors to focus correctly on faraway galaxies is another challenge. Actuators, or tiny mechanical motors, provide the answer to achieving a single perfect focus. The primary mirror segments and secondary mirror are moved by six actuators that are attached to the back of each mirror piece. The primary mirror segments also have an additional actuator at its center that adjusts its curvature. The telescope's tertiary mirror remains stationary.

Lee Feinberg, Webb Optical Telescope Element Manager at NASA's Goddard Space Flight Center in Greenbelt, Md. explained, "Aligning the primary mirror segments as though they are a single large mirror means each mirror is aligned to 1/10,000th the thickness of a human hair. What's even more amazing is that the engineers and scientists working on the Webb telescope literally had to invent how to do this."

Watch the the actuators being attached to the back of a telescope mirror in this "Behind the Webb" video:

One further challenge is to keep Webb's mirror cold. To see the first stars and galaxies in the early Universe, astronomers have to observe the infrared light given off by them, and use a telescope and instruments optimized for this light. Because warm objects give off infrared light, or heat, if Webb's mirror was the same temperature as the Hubble Space Telescope's, the faint infrared light from distant galaxies would be lost in the infrared glow of the mirror. Thus, Webb needs to be very cold ("cryogenic"), with its mirrors at around -220 degrees C (-364 degree F). The mirror as a whole must be able to withstand very cold temperatures as well as hold its shape.

To keep Webb cold, it will be sent into deep space, far from the Earth. Sunshields will shade the mirrors and instruments from the Sun's heat, as well as keep them separated from the warm spacecraft bus.

How Did NASA Come Up With These Ideas?

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, one being that beryllium holds its shape at cryogenic temperatures. Based on the expert team's recommendation, Northrop Grumman (the company that is leading the effort to build Webb) selected a beryllium mirror, and the project management at NASA's Goddard Space Flight Center approved this decision.

Why Beryllium?

a marble sized piece of beryllium

Beryllium is a light metal (atomic symbol: Be) that has many features that make it desirable for Webb's 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 Mirrors Were Made

The James Webb Space Telescope's 18 special lightweight beryllium mirrors have to make 14 stops to 11 different places around the U.S. to complete their manufacturing. They come to life at beryllium mines in Utah, and then move across the country for processing and polishing. In fact, the mirrors make stops in eight states along the way, visiting some states more than once, before journeying to South America for lift-off and the beginning of their final journey to space. Explore an interactive map showing the journey of the mirrors.

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

Beryllium Mirror Segments

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

Axsys Technologies shaped the mirror blanks into their final shape. 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.

Back of the mirror

The front surface of each blank was smoothed out and shaped properly so that it will be ready for its final position in the large mirror.

Mirror Blank

This movie shows the mirror blanks being made at Brush Wellman and shaped at Axsys.

Once the mirror segments were shaped by Axsys, they were sent to Richmond, CA, where SSG/Tinsley polished them.

Mirror Blank

SSG/Tinsley started by grinding down the surface of each mirror close to its final shape. After this was done, the mirrors were carefully smoothed out and polished. The process of smoothing and polishing is repeated until each mirror segment is nearly perfect. At that point, the segments travel to NASA's Marshall Space Flight Center in Huntsville (MSFC), Alabama for cryogenic testing.

Since many materials change shape when they change temperature, a test team from Ball Aerospace worked together with NASA engineers of Marshall Space Flight Center’s X-ray and Cryogenic Facility (XRCF) to cool the mirror segments down to the temperature Webb will expericence in deep space, -400 degrees Farenheit (-240 degrees Celsius).

Cryogenic testing of the primary mirror segments began in at Marshall's XRCF by Ball Aerospace in 2009.

Ball Aerospace Ball Aerospace

The change in mirror segment shape due to the exposure to these cryogenic temperatures was recorded by Ball Aerospace Engineers using a laser interferometer. This information, together with the mirrors, traveled back to California for final surface polishing at Tinsley.

This short video shows part of the mirror polishing process. The mirrors' final polish was completed in June of 2011.


You can learn more about how the mirror segments are polished in this "Behind the Webb" video podcast:


Once a mirror segment's final shape is corrected for any imaging effects due to cold temperatures, and polishing is complete, a thin coating of gold is applied. Gold improves the mirror's reflection of infrared light.

gold-coated mirrors

Below is the engineering design unit primary mirror segment (flight spare) coated in gold by Quantum Coating Incorporated. Photo by Drew Noel.

gold-coated primary mirror EDU

After the gold coating was applied, the mirrors once again traveled back to Marshall Space Flight Center for a final verification of mirror surface shape at cryogenic temperatures. The mirror segments are now complete - they will soon travel to NASA's Goddard Space Flight Center in Greenbelt, Maryland.

gold-coated primary mirror EDU

The secondary mirror, now completed, went through a similar process - here it is after being gold-coated by Quantum Coating Incorporated.

gold-coated primary mirror EDU

In this video, you can follow the mirror's journey from rough ore to precisely reflective, gold-coated segments:

Assembling the Telescope

ITT (formerly Kodak) will combine 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, will be sent to Goddard. ITT will mount 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. It is these wings that fold back so that the full mirror will fit into a rocket.

Learn More

View our mirror image gallery and our mirror video archive.