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December
10, 2008: The James Webb Space Telescope, targeted
for launch in 2013, is already taking an incredible journey
right here on Earth. It's zigzagging up, down, and across
the US to be "spit and polished" to perfection for
its lofty space mission.
 "To
find the first stars and galaxies that formed in the early
universe, which are millions and even billions of light years
away, the Webb telescope mirror has to be wickedly smooth,"
says Jeff Kegley of NASA's Marshall Space Flight Center.
Right:
The James Webb Space Telescope, an artist's concept.
To
get ready for space, the 18 mirror segments that will ultimately
form the Webb telescope’s huge primary mirror are trucked
from pit stop to pit stop in tandem cross-country for careful
processing and polishing. They visit seven different states,
some several times.
During
the long odyssey, every precaution
is taken for their protection. How many years of bad luck
would you have if you broke one of these mirrors?
"That's
something we don't talk about," laughs Helen Cole, also
of Marshall. "But seriously, JWST has 3 mirror segments
on the back burner for use if needed as spares."
Let's
trace a mirror segment's Earthly journey from rough start
to "wickedly smooth," and finally to union with
its 17 siblings to form a 6.5 meter (21 ½ foot) wide whole
with a total area of 25 square-meters (almost 30 square yards).
The
story begins in a Utah beryllium mine. Beryllium is one of
the lightest of all metals, and the "stuff" of the
telescope's mirrors.
Above:
The making of the JWST mirrors begins here in a Utah Beryllium
mine. Photo credit: Brush Wellman, Inc., Beryllium Products
division. [Larger image]
Technicians
in Ohio sift and purify the gritty beryllium powder from Utah
into an extremely uniform optical grade especially for the
Webb mirror. Then they pour the powder in a big, flat can,
apply heat and pressure, and pump out the residual gas to
create a large slab called a mirror billet. They bathe the
billet in acid to burn off any stainless steel stuck to the
billet when the can is removed. Next they split the billet
in half Oreo-cookie-style to form two mirror blanks (no cream!).
These mirror blanks are the largest ever produced in beryllium.
Workers
in Alabama machine the back of each blank into a honeycomb
structure to make the blanks lighter without reducing stiffness.
The machined ribs are less than 1 millimeter thick -- almost
paper cut thin!
"This
precision machining/etching removes 92 percent of a blank's
mass," says Lee Feinberg of the Goddard Space Flight
Center. "Mass is critical in launching space missions."
Next,
a California company grinds and polishes the segments to a
very smooth and exact shape and optically tests them at room
temperature.
Above:
Key stops in the long journey of the JWST. Not shown: space.
[Larger image]
But
the Webb telescope will not operate in room temperature. Not
only will this telescope mirror be "wickedly smooth,"
it will also be wickedly cold in space. Because it is an infrared
telescope, the JWST is designed to pick up the heat of faint,
awesomely distant stars and galaxies. To do that it has to
be kept extremely cold. It will operate in space at about
-238 deg Celsius (-396 deg Fahrenheit, 35K).
"The
extreme cold may cause the telescope's structures and mirrors
to change shape, so testing has to be done here on Earth under
similar, hyper-cold conditions," says Cole.
This
super-cold testing is done in Alabama. The Marshall Space
Flight Center's X-ray & Cryogenic Facility has a vacuum
chamber that can simulate the incredibly cold conditions of
space. Testing in this chamber reveals even the tiniest distortions
that happen to the mirror segments in the cold. The tests
provide precise data that specifies the exact repolishing
to be done to compensate ahead of time for distortions likely
to occur in space.
Above:
(Left) A prototype JWST beryllium mirror
segment at Tinsley Labs in Richmond, California; (Right)
Mirror testing under space-cold conditions at the Marshall
Space Flight Center's X-ray & Cryogenic Facility. [Larger
images: #1,
#2]
Once
the mirror segments are polished to precision, gold is evaporated
over them, forming a very thin coating on the smooth mirror
surface.
"This
gold coating is highly reflective over all the wavelengths
of the Webb telescope, from visible to mid-infrared,"
says Feinberg.
All
18 segments finally meet at Goddard Space Flight Center. Here,
they're mounted on structures that will ultimately hold them
in place and let them perform as if they were part of a single
giant hexagonal mirror. (The mirror structure will be folded
with its shield origami style when it's time to fit in a rocket.)
Next the telescope is fully assembled and attached to the
instrument module, and the whole kit and caboodle is acoustic
and vibration tested.
Final
cryogenic testing takes place at Johnson Space Center, in
the same vacuum chamber that tested the Apollo lunar lander.
The
telescope is integrated with the spacecraft and sunshield
at Northrop Grumman in California. It will lift-off from Kourou,
French Guiana, on an Ariane 5 rocket.
Are
we there yet? Almost. Only 930,000 more miles to go....
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Author: Dauna Coulter
| Editor: Dr.
Tony Phillips | Credit: Science@NASA
| more
information |
|
Credits:
The Webb telescope is a joint project of NASA and many
U.S. partners, the European Space Agency and the Canadian
Space Agency. Ball Aerospace & Technologies Corp.
is the principal optical subcontractor for the Webb telescope
program, led by prime contractor Northrop Grumman Space
Technology, under a contract from the NASA Goddard Space
Flight Center, in Greenbelt, Maryland. Ball Aerospace
is in charge of the optical testing at the Marshall Center,
which is providing a facility for the testing and supporting
Ball, helping to ensure an appropriate and safe environment
for testing that meets the needs of the project.
For
more information about the James Webb Space Telescope,
please visit: http://jwst.gsfc.nasa.gov
In Detail:
Webb Telescope Mirror Fabrication and Processing Stops
Elmore,
Ohio
Brush Wellman, Inc.
Sifts and purifies the gritty Beryllium powder into
an extremely uniform optical grade especially for the
Webb mirror. Creates mirror blanks -- the largest ever
produced in Beryllium.
Cullman,
Alabama
Axsys Technologies, Inc.
Machines the back of each blank into a honeycomb structure
to make the blank lighter without reducing its stiffness.
Also machines the mirror fronts for general shaping.
Richmond,
California
L3 Communications, Tinsley Laboratories
Grinds and polishes the segments to a very smooth and
exact shape, then optically tests them at room temperature.
Boulder,
Colorado
Ball Aerospace & Technologies Corp.
Attaches mount and actuators to the mirror segments,
performing vibration and optical testing before and
after assembly.
Huntsville,
Alabama
The X-ray Cryogenic Facility, or XRCF
Tests the segments in the XRCF vacuum chamber. Relays
findings back to Tinsley Labs, arming them with precise
numbers indicating the exact extra polishing needed
to compensate ahead of time for distortions likely to
occur in space.
Boulder,
Colorado
Ball Aerospace & Technologies Corp.
Removes mount and actuators.
Richmond,
California
L3 Communications, Tinsley Laboratories
Fine-tunes each mirror piece by polishing in the opposite
of the surface error values derived from the XRCF's
super-cold testing. Now, distortion that occurs in space-cold
should actually perfect the mirror.
Boulder,
Colorado
Ball Aerospace & Technologies Corp.
Cleans mirror segments to prepare for coating.
Moorestown,
New Jersey
Quantum Coating, Inc. -- Denton
Evaporates gold over the segments in a huge vacuum chamber,
forming a very thin coating on the smooth mirror surface.
Boulder,
Colorado
Ball Aerospace & Technologies Corp.
Reassembles the mirror segments with mount and actuators
and performs final acceptance vibration testing.
Huntsville,
Alabama
The X-ray & Cryogenic Facility
Performs final cryogenic acceptance testing on the segments.
Greenbelt,
Maryland
Goddard Space Flight Center
Assembles the telescope and attaches it to the instrument
module. Performs acoustic and vibration tests.
Houston,
Texas
Johnson Space Center
Final cryogenic testing.
Redondo
Beach, California
Northrop Grumman
Integrates the assembly with the spacecraft and sunshield.
Kourou,
French Guiana
Guiana Space Centre
Lift-off!
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