Mirrors will be very small, move independently

Sandia-developed micromirrors, each slightly larger than a cross section of a human hair, may one day be part of the Next Generation Space Telescope (NGST), the successor to the Hubble that will peruse the universe looking for remnants from the period in which the first stars and galaxies formed.

"We are designing mirrors that will be very, very small, move independently, and be able to withstand the very cold temperatures and extreme conditions of space," says Ernie Garcia (2643), the engineer leading the mirror development effort.

The mirrors will be sensitive to infrared radiation and, as a result, will be able to detect faint signals from the first billion years after the Big Bang. This will help scientists better understand the origins of the universe. Aware of Sandia’s advancements in microelectromechanical systems (MEMS) technology, NASA approached the Labs last year about developing prototype MEMS mirrors that could be part of the NGST, which is tentatively scheduled for launch in 2008. The one-year contract began in January, and nine months later Ernie had functioning mirrors to show the agency.

The fast turnaround of Sandia’s Microelectronics Development Laboratory, which fabricated the mirrors, made it possible for Ernie to provide NASA with a working device so quickly, he says.

In September Ernie demonstrated to NASA/Goddard Space Flight Center in Greenbelt, Md., an array of working mirrors, each 100 microns by 100 microns with one micron gaps between adjacent mirrors, lined up in rows of three. Each row tilted 10 degrees in unison — a large angle for this design.

"Getting these miniaturized mirrors to rotate to such a large angle was a real milestone in the research," Ernie says. "It’s something that NASA wanted, and we did it."

The goal is to have four million of these independently moving mirrors in the NGST. Each mirror could be tilted in different directions to redirect optical signals to an infrared detector.

In light of the success of getting the mirrors to rotate at large angles, Ernie says he is hopeful that NASA will extend the contract to continue the research.

NASA is pursuing the NGST as the successor to the Hubble Space Telescope in an effort to observe the "Dark Zone," a period 100 million to one billion years after the Big Bang when primordial seeds began to evolve into the galaxies and stars known today. It would also see formations in the present day universe. The Hubble has provided data about more recent formations, but has been unable to detect the earlier stars that fall in the infrared range because it was designed as an optical telescope.

The NGST, on the other hand, will be extremely sensitive to infrared radiation, and with its large light-gathering mirror and superb resolution, will be capable of detecting the earlier signals. The new telescope will be placed in orbit well beyond the Earth’s moon to reduce stray light and achieve the cold temperatures needed to observe in the infrared.

Currently three entities– Lockheed Martin, Goddard Space Flight Center, and TRW — are studying different design approaches for the NGST. Each approach includes adjustable thin mirrors, deep space orbits, fast-steering mirrors for fine guidance, and infrequent contact with the ground. They differ in the areas of mirror construction, materials and deployment, detector types, sunshield types, vibration control and launch vehicles.

Eventually NASA will select one design from the three for the final NGST.

The mirrors Ernie is designing could go into any one of the three NGST approaches as part of the Integrated Science Instrument Module that will also include cameras, spectrographs, and infrared detectors.

The micromirrors will work in conjunction with a very large mirror — possibly eight meters in diameter — that will collect light from a broad area in space. When an object is encountered that appears interesting, the smaller micromirrors would be tilted to reflect the image from only that area, beaming the information to an infrared detector.

Ernie says he still faces several challenges in developing moving mirrors for the NGST.

One is making the mirrors able to function in extremely cold temperatures.

"Instrument operating temperatures in space can be 30 degrees K [-405 degrees F] or lower," Ernie says. "That means we have to build these mirrors a special way so that they won’t break at such extremes."

The mirrors are built by depositing thin films of polycrystalline silicon on a silicon wafer. The first layer, called poly0, contains connection wires. The others, poly1, poly2, and poly3, are mechanical layers that allow the MEMS device to move. In the near future Ernie plans to add on top of the poly3 a final thin layer of gold to reflect infrared light.

Therein lies the problem in cold temperatures, Ernie says.

"Different materials shrink at different rates when subjected to temperature changes," Ernie says. "As the temperature is reduced, the gold layer will shrink faster than the polysilicon. This will cause stress. If the stress levels get too high, the mirror could break or deform or the gold could peal away. We have to come up with the smallest thickness of gold so that it doesn’t cause excessive stress, but yet be thick enough to reflect the infrareds."

The MDL will soon begin fabricating an improved mirror design, which will be cold-tested by NASA early next year in a cryogenic chamber where the conditions of deep space will be simulated.

"After that test we will see what problems we have and then start to figure out how to fix them," Ernie says. Another challenge Ernie is concurrently striving to resolve is that of making each of the mirrors move independently. The new design, which will soon be fabricated at the MDL, has each row tilting in unison and one mirror in the middle tilting and moving independently. But doing this for each of the four million mirrors is a major hurdle.

"Addressing each mirror is really a tough problem," he says. "Now that we only have an array of nine mirrors, we can easily connect each one with wire. The question is where are we going to find physical space for wires to connect four million mirrors. We may have to come up with an alternate way of connection."