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.
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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.
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| 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?
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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)
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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 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.
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.
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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!
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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.
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We have a piece on JWST's mirrors in our technology section here. |
