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Mirrors completed for successor to Hubble telescope

Made by Ball Aerospace, the 18 beryllium mirror segments for the James Webb Space Telescope are ready to be delivered to NASA. 

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One of the most challenging parts of NASA’s huge new space telescope, the building of its ultrasophisticated mirror system, is now finished, and the mirrors are ready for delivery.

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Send-off ceremonies held here at Ball Aerospace on Aug. 15 saluted the completion of 18 beryllium primary mirror segments for the James Webb Space Telescope (JWST), which is billed as the successor to NASA's venerable Hubble Space Telescope. Ball is also responsible for JWST’s secondary and tertiary mirrors, a fine steering mirror assembly and several engineering development units.

Ball is the principal subcontractor to manufacturer Northrop Grumman for the JWST optical technology and lightweight mirror system at the heart of the telescope — an astronomical project that is now pegged to cost roughly $8.7 billion and to be lofted in the fall of 2018.

Success story

The JWST mirror system includes 18 gold-coated, ultrasmooth, 4.2-foot (1.3 meters) hexagonal mirror segments that comprise the 21.3-foot (6.5 m) primary mirror. When launched, it will be the largest mirror ever flown in space. [Photos: Building the James Webb Space Telescope]

Down on the floor where the packaged mirrors are ready for shipping to NASA, labels such as "do not stack…this side up" and "critical space flight hardware" are visible.

A folding scheme allows the primary mirror segments to fit atop Europe’s Ariane 5 launcher for their eventual unfolding in space. Aligning the mirror segments and adjusting the primary mirror’s curvature will occur over approximately two months.

It has taken about eight years to complete the fabrication of the mirrors, said Paul Lightsey, a Ball mission systems engineer for the optical system on JWST.

"We actually have a real nice success story," Lightsey told SPACE.com. "We’ve been able to show how long it took to polish the first mirror, then each successive mirror. By the time we got up to the later mirrors, we were taking half the time than it took for the first mirror."

Working together as one mirror, those 18 beryllium mirror segments are adjusted by computer-controlled actuators. They adjust each of the mirror segments to correct any errors and are key to giving JWST the power to produce high-quality, sharp images.

"One of the difficulties in making mirrors is to make the curvature exactly what you want," Lightsey said. JWST's mirrors can be pushed and pulled a little to get the curvature right, as well as moved up, down and sideways, he said.

Lifetime at L2

Allison Barto, JWST program manager at Ball Aerospace, said the beryllium mirrors couldn’t be too heavy.

"We had to take out over 90 percent of the material in the back of the mirrors to make them light enough to launch 18 of them into space," she said.

Since JWST is an infrared telescope, the mirrors and actuators must function at temperatures as low as minus 400 degrees Fahrenheit (minus 240 degrees Celsius).

Lightsey said the JWST project is set to be a five-year mission, but has a goal of 10 years beyond commissioning. Outfitted with a five-layer sunshield, JWST will operate at supercold temperatures at a spot about 930,000 miles (1.5 million kilometers) from Earth known as Lagrange Point 2, or L2.

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