The Primary Mirror
An Overview
One of the James Webb Space Telescope's science goals is to look backwards in 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, nor has a mirror this large ever been launched into space before!
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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.
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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.
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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?
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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)
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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 JWST 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.
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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. |
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The front surface of each blank is smoothed out and shaped properly so that
it will be ready for its final position in the large mirror.
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Watch a movie showing how the mirror blanks being made at Brush Wellman and shaped at Axsys! |
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Once the mirror segments were shaped by Axsys, they were sent to Richmond, CA, where SSG/Tinsley
is polishing them.
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SSG/Tinsley starts by grinding down the surface of each mirror close to its final shape. After this is done, the mirrors are carefully smoothed out and polished. This process of smoothing and polishing is repeated until the whole mirror segment is nearly perfect. At that point, the segments travel to NASA's Marshall Space Flight Center in Huntsville (MSFC), Alabama.
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Since many materials change shape when they change temperature, a test team from Ball Aerospace works 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, -220 degree Celsius (-364 degree F). The change in mirror segment shape due to the exposure to these cryogenic temperatures is recorded by Ball Aerospace Engineers using a laser interferometer. This information, together with the mirrors, travel 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 is corrected for any imaging effects due to cold temperatures, a thin coating of gold will be applied. Gold improves the mirror's reflection of infrared light. After the gold coating is applied, the mirrors once again travel back to Marshall Space Flight Center for a final verification of mirror surface shape at cryogenic temperatures. Once a mirror segment complete, it travels to NASA's Goddard Space Flight Center in Greenbelt, Maryland.
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.
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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 the heat, 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.
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"The James Webb Space Telescope will collect light approximately 9 times faster than the Hubble Space Telescope when one takes into account the details of the relative mirror sizes, shapes, and features in each design," said Eric Smith, Webb program scientist at NASA Headquarters, Washington. The increased sensitivity will allow scientists to see back to when the first galaxies formed just after the Big Bang.The larger telescope will have advantages for all aspects of astronomy and will revolutionize studies of how stars and planetary systems form and evolve.
