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UNITED STATES
US: Mirror coating of NASA telescope
NASA's James Webb Space Telescope, the world's next-generation space observatory and successor to the Hubble Space Telescope, has reached a major milestone in its development: the mirrors that will fly aboard the telescope have been completely coated with a microscopically thin layer of gold, selected for its ability to properly reflect infrared light from the mirrors into the observatory's science instruments.

The most powerful space telescope ever built, the Webb will provide images of the first galaxies ever formed and explore planets around distant stars. It is a joint project of NASA, the European Space Agency and the Canadian Space Agency. The coating allows the telescope's 'infrared eyes' to observe extremely faint objects in infrared light. Webb's mission is to observe the most distant objects in the universe.

"Finishing all mirror coatings on schedule is another major success story for the Webb telescope mirrors," says Lee Feinberg, NASA Optical Telescope Element manager for the Webb telescope at the agency's Goddard Space Flight Center. "These coatings easily meet their specifications, ensuring even more scientific discovery potential for the telescope."

The telescope has 21 mirrors, with 18 mirror segments working together as one large 6.5-metre primary mirror. The mirror segments are made of beryllium, which was selected for its stiffness, light weight and stability at cryogenic temperatures.

Bare beryllium is not very reflective of near-infrared light so each mirror is coated with about 0.12 ounce of gold. The last full size 1.5-metre hexagonal beryllium primary mirror segment that will fly aboard the observatory recently was coated, completing this stage of mirror production.

Mirror manufacturing began eight years ago with blanks made out of beryllium, an extremely hard metal that holds its shape in the extreme cold of space where the telescope will orbit. Mirror coating began in June 2010 when several of the smaller mirrors in the telescope, the tertiary mirror and the fine steering mirror, were coated. The secondary mirror was finished earlier this year.

Quantum Coating Inc. constructed a new coating facility and clean room to coat the large mirror segments. The company also developed the gold coating for performance in certain areas, such as uniformity, cryogenic cycling, durability, stress and reflectance, in a two-year effort prior to coating the first flight mirror.

In the process, gold is heated to its liquid point, more than 1,371 degrees C, and evaporates on to the mirror's optical surface. The coatings are 120 nanometers, a thickness of about 200 times thinner than a human hair.

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