David Baiocchi, Jim Burge and Brian Cuerden have taken an innovative space-mirror technology originally developed at the Optical Sciences Center and Steward Observatory for the Next Generation Space Telescope (NGST) to new limits.

The new ultralight half-meter (21-inch) NRO mirror was developed with funds from the National Reconnaissance Organization (NRO). It weighs only a kilogram (2.2 pounds).

Geosynchronous orbit is the ideal place from which to observe Earth. The real advantage is that a mirror in geosynchronous orbit 22,241 miles above Earth remains fixed over its target, Baiocchi said. And it could observe about half the planet, so fewer satellites are needed to cover the globe than are needed at low earth orbit.

But to get the same resolution as a satellite in low earth orbit, a telescope in geosynchronous orbit would have to be 100 times larger. "And you can't do this with current technology," Baiocchi added.

Their NRO-funded research shows that giant, extremely lightweight active mirrors might be feasible for future space telescopes, Burge said.

Burge heads the UA's 2-meter NGST prototype mirror project known as the Next Generation Space Telescope Mirror System Demonstrator (NMSD). He and others at UA Steward Observatory Mirror Lab began working with active mirror technology in 1996. NASA's NGST, a powerful space telescope being designed to replace the Hubble Space Telescope when it retires near the end of this decade, is scheduled for launch in 2009. The UA scientists' NMSD system is currently being assembled in the optics shop at the Optical Sciences Center.

"We used the 2-meter (approximately 6.5 feet) NGST prototype mirror as a starting point for our project," Baiocchi said. "Then, everywhere we could possibly trim weight, we did."

"The NRO mirror represents proof of principle," Burge said. "If you can make this one, you can make another. We're taking a technology we're already familiar with and then pushing it, making a mirror as lightweight as possible. And that's lighter than anything anyone else is doing. It's really important for the NGST and for future space optics that have to go lighter."

The NRO mirror features an aluminum-coated reflective glass facesheet only one millimeter thick, that is, less than four-hundredths of an inch. The shape of the glass is governed by 31 tiny, computer-controlled actuators, each about the size of a quarter and weighing 5 grams, which is less than a fifth of an ounce, or a hundredth of a pound.

Triangular-footed "loadspreaders" glued to the back of the glass and coupled with the actuators minimize the force at the contact points. The system and actuators are mounted on a rigid, lightweight carbon fiber support structure.

"In a space mirror, we're trying to hold the glass to a perfect shape. We attach it to actuators that slowly control the mirror," Burge said.

The surface of the mirror is so accurate that if the 21-inch mirror were the size of a football field, the difference between the highest and lowest points would be the thickness of two human hairs, Baiocchi said. The mirror shows only the effects of gravity, which would be released when the mirror is launched into space.

As they did in making the 2 millimeter thick NGST prototype mirror, the researchers fabricated the 1 millimeter thick NRO glass facesheet starting with a thick, six-inch slab of a special glass called Zerodur. They first polished the optical surface. Then they glued the glass to a granite block of the desired curvature using pitch (pine tar). Finally, they machined off the extra glass and attached loadspreaders to the facesheet while it was still supported on the granite block.

They deblocked the mirror by heating everything in a 400-degree Fahrenheit oven to soften the pitch, so they could gently slide the glass off intact.

The NGST prototype mirror, unlike the NRO mirror, was engineered for launch. The next step for ultra lightweight NRO-type mirrors would be to make a half-meter mirror for actual flight testing, the scientists said.

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