Mirror Segments Exposed To Space-Like Conditions Prior To
The first six of 18 segments that will form NASA's James Webb
Space Telescope's primary mirror for space observations began final
round-the-clock cryogenic testing this week. These tests will
confirm the mirrors will respond as expected to the extreme
temperatures of space prior to integration into the telescope's
permanent housing structure.
The X-ray and Cryogenic Facility at NASA's Marshall Space Flight
Center in Huntsville, AL, will provide the space-like environment
to help engineers measure how well the telescope will image
infrared sources once in orbit. Each mirror segment measures
approximately 4.3 feet in diameter to form the 21.3 foot, hexagonal
telescope mirror assembly critical for infrared observations. Each
of the 18 hexagonal-shaped mirror assemblies weighs approximately
88 pounds (40 kilograms). The mirrors are made of a light and
strong metal called beryllium, and coated with a microscopically
thin coat of gold to enable the mirror to efficiently collect
"The six flight mirrors sitting ready for cryogenic acceptance
tests have been carefully polished to their exact prescriptions,"
said Helen Cole, project manager for Webb activities at Marshall.
"It's taken the entire mirror development team, including all the
partners, over eight years of fabrication, polishing and cryogenic
testing to get to this point."
During cryogenic testing, the mirrors are subjected to extreme
temperatures dipping to minus 415 degrees Fahrenheit (-248C) in a
7,600 cubic-foot (approximately 215 cubic meter) helium-cooled
vacuum chamber. This permits engineers to measure in extreme detail
how the shape of the mirror changes as it cools. This simulates the
actual processes each mirror will undergo as it changes shape over
a range of operational temperatures in space.
"This final cryotest is expected to confirm the exacting
processes that have resulted in flight mirrors manufactured to
tolerances as tight as 20 nanometers, or less than one millionth of
an inch," said Scott Texter, Webb Optical Telescope element manager
at Northrop Grumman in Redondo Beach, CA.
A second set of six mirror assemblies will arrive at Marshall in
July to begin testing, and the final set of six will arrive during
The Webb Telescope is NASA's next-generation space observatory
and successor to the Hubble Space Telescope. The most powerful
space telescope designed, Webb will observe the most distant
objects in the universe, provide images of the very first galaxies
ever formed and help identify unexplored planets around distant
stars. The telescope will orbit approximately one million miles
"The Webb telescope continues to make good technological
progress," said Rick Howard, JWST Program Director in Washington.
"We're currently developing a new baseline cost and schedule to
ensure the success of the program."