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!
| 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 1.3 meters (4.26 feet) in diameter.
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.
|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 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.
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.|
|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.|
Once the mirror segments were shaped by Axsys, they were sent to Richmond, CA, to be polished by SSG/Tinsley.
|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. This process of smoothing and polishing was repeated until the whole mirror segment is nearly perfect. At that point, the segments traveled to NASA's Marshall Space Flight Center in Huntsville (MSFC), Alabama.|
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 experience in deep space, -400 degrees Farenheit (-240 degrees Celsius). 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. Cryogenic testing of the primary mirror segments began in at Marshall's XRCF by Ball Aerospace in 2009.
Once the mirror segment's final shape was corrected for any imaging effects due to cold temperatures, a thin coating of gold was applied. Gold improves the mirror's reflection of infrared light.
Below is the engineering design unit primary mirror segment coated in gold by by Quantum Coating Incorporated. Photo by Drew Noel.
The secondary mirror, now completed, went through a similar process - here it is after being gold-coated by Quantum Coating Incorporated.
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.
Want to know more? Visit the James Webb Space Telescope project home page.