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Another Test for Mirrors on Biggest Space Telescope

The mirrors that Dr. James Hadaway and his associates will soon test for NASA's next great space observatory look pretty much like he thought they should look 15 years ago.

Scheduled to arrive at NASA's Marshall Space Flight Center on Monday, July 25, six gold-coated mirrors for the James Webb Space Telescope (JWST) must be tested under extreme conditions to ensure that the mirrors will be smooth and focused when they are put to work most of a million miles from Earth.

A principal research scientist at The University of Alabama in Huntsville's Center for Applied Optics, Hadaway leads the optical testing of the Webb telescope's 18 primary mirrors. Subcontractors through Ball Aerospace, he and Dr. Patrick Reardon are part of a team working to ensure that the Webb telescope sees everything it should see. The observatory was designed to look at stars and galaxies on the distant edges of the universe.

The mirrors arriving next week are the second of three sets. The first six mirrors all passed their tests in April and May, despite Marshall Space Flight Center (MSFC) losing electric power following an outbreak of tornadoes in late April.

"We had six mirrors in the vacuum chamber and we were at the cold temperature -- 45 Kelvin (about 378° F below zero) -- and we had finished measurements on three mirrors by that Wednesday afternoon," recalled Hadaway. "It was a pretty critical time. It takes a lot to get and keep these mirrors cold.

"We knew something bad was happening outside, but we had to focus on what we were doing."

UAHuntsville's optical measurement team headed for home about 5:30 that afternoon, just before the power went out at MSFC. That included power to the X-Ray and Cryogenic Facility (XRCF) where the mirror testing is done.

"Fortunately, the XRCF has a diesel generator to keep things running in a power outage," Hadaway said. "That night they e-mailed us, 'We're still good,' so for the next three days we were out there taking data. They only had enough power for the instruments and to maintain the chamber, so by Saturday it was up to 85 inside the building. But we ended up getting all of our data as planned, on budget and on schedule.

"The real story on this is the people who run the facility. They're the ones who did the heroic job, arranging for enough diesel fuel, liquid nitrogen, liquid helium and all the other things needed to keep us running. They did a great job of keeping everything going."

Once they are cooled to the temperature of deep space, the extraordinarily smooth mirrors have to be warmed slowly over a period of several days to avoid damage, distortion or condensation, which could leave behind deposits on the polished gold surface.

Hadaway has been part of the Webb telescope program from its beginning, when he led the optical design team that came up with the initial layout for the telescope. More than two and a half times bigger than the Hubble space telescope, the Webb will collect infrared radiation (energy that our bodies sense as heat) from the most distant stars and galaxies ever viewed.

"The final optical design is basically the same as my original design," Hadaway said. "The optics weren't too difficult to design. The hard part was making lightweight mirrors that will survive launch loads and then deploy properly."

The next step would be building and testing mirrors, starting with engineering mockups and continuing with flight hardware.

What would have been routinely challenging was complicated by the observatory's working environment. Infrared energy includes the same wavelengths created when sunlight warms a spacecraft. To avoid polluting weak infrared radiation from galaxies on the edges of the universe with heat absorbed from sunlight, the Webb telescope will be shaded from the sun. Sitting in that shade, the mirrors will operate at temperatures about 45 degrees Celsius above absolute zero.

When the mission was proposed, no one knew how telescope mirrors built on Earth at room temperatures might bend and distort at temperatures that cold. During a meeting at NASA's Marshall Space Flight Center, officials wondered where they might find someone with the special knowledge and skills needed to organize and conduct a mirror testing program under those extreme conditions.

Hadaway stuck up his hand.

"We can do that," he said. After working with NASA to develop specialized mirrors used for X-ray telescopes, Hadaway was confident the CAO team could develop the tools needed to test mirrors designed to collect energy at the other end of the electromagnetic spectrum.

A house-sized cryogenic chamber in the XRCF, a facility built for testing the Chandra X-ray Observatory's optics, was adapted for testing Webb mirrors at extremely cold temperatures.

Using techniques that they developed, "we measured to see how each mirror deforms when it 'goes cold,'" Hadaway said. "We send what we find to Tinsley Laboratories in Richmond, California, which polishes opposite distortions into the mirrors. If it was a bump when it was cold, they polish in a hole. Now it looks bad at room temperature, but it's perfect in the cold."

Perfect? The average imperfection allowed is the height of about 200 hydrogen atoms.

Testing mirrors in a massive insulated chamber requires Hadaway and his team to be flexible. "If it gets cold at 3 a.m., you go in at 3 a.m.," he said. "You go when they're cold. We try to work with the guys at Marshall so things work out in the daytime, but you just have to be there when it's time."

The mirror-testing program is scheduled to end by the end of this year, although Hadaway expects to be involved in the Webb telescope's ongoing testing, development and preparations for a possible launch in 2015. In the 15 years he has been involved in the program, Hadaway's team has received more than $5 million in NASA funding to support UAHuntsville's work.

"I was part of this program from day one," he said. "My goal is to be there when it's on orbit and certified to be operational."

Dr. James Hadaway, (256) 824-2533
Ray Garner

Ray Garner | Newswise Science News
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