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!
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.)
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.
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:
These diagrams show the back of the mirrors and the actuators.
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.
Here is an animation of how light travels through the telescope. JWST is what is known as a three mirror anastigmat. In this configuration, the primary mirror is concave, the secondary is convex, and it works slightly off-axis. The tertiary removes the resulting astigmatism and also flattens the focal plane. This also allows for a wider field of view.
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 (formerly ITT, now the Harris Corporation) 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
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
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. This makes each segment very light. A beryllium mirror segment is 20 kilograms in mass. (A full primary mirror segment assembly including its actuator is about 40 kg.)
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.
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
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.
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.
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.
Some Technical Details: How is the gold applied to the mirrors? The answer is vacuum vapor deposition. Quantum Coating Incorporated did
the coatings on our telescope mirrors. Essentially, the mirrors are put inside a vacuum chamber and a small quantity of gold is vaporized and it deposits on the mirror. Areas that we don't want coated (like the backside and all the mechanisms and such) are masked-off. Typical thickness of the gold is 1000 Angstroms (100 nanometers). A thin layer of amorphous SiO2 (glass) is deposited on top of the gold to protect it from scratches in case of handling or if particles get on the surface and move around (the gold is pure and very soft).
This Behind the Webb video is about mirror coating:
Below is the engineering design unit primary mirror segment (flight spare) coated in gold by Quantum Coating Incorporated. Photo by Drew Noel.
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.
The secondary mirror, went through a similar process - here it is after being gold-coated by Quantum Coating Incorporated.
In this video, you can follow the mirror's journey from rough ore to precisely reflective, gold-coated segments:
The Assembled Mirrors
Here are photos of the assembled mirrors, and of the assembled OTIS - that is, the Optical Telescope Element and Integrated Science Instrument Module.
Below, mirror assembly - note the protective black covers on the mirrors: