MEMS for Viewing Faraway Galaxies
NASA engineers forge a next-generation microshutter device that allows the viewing of distant infrared images.
Efforts to peer deep into the early universe are important to understanding its formation, but this requires the gathering of very faint infrared light–which is difficult because nearer, brighter objects overwhelm the signals of darker objects that are farther away. Now engineers at NASA have designed a highly sensitive device with 62,000 micrometer-scale shutters that allow scientists to choose objects they wish to study and block light from other objects.
The new microshutter system is destined for the James Webb Space Telescope, scheduled to replace the Hubble Space Telescope in 2013. It starts with a piece of specially made silicon that includes a 38-by-38-millimeter area of shutters that sit atop a camera, called the Near Infrared Spectrograph, being built by the European Space Agency (ESA).
Scientists will guide the opening and closing of the microshutters by first examining images taken from an infrared camera onboard the telescope, says Harvey Moseley, the microshutter principal investigator. Scientists viewing the images will select the distant objects they want to spectrograph. A computer system with a digital map will coordinate the opening and closing of the shutters. “It’s a bit like driving west into the sunset, and you pull the visor down on your car,” says Moseley. “You remove a lot of light from your detection system [field of view], and it vastly improves sensitivity. This is the motivation behind the microshutters–eliminating all unnecessary light.”
The microshutters will allow scientists to peer farther into space than ever before by completely blocking the signals of bright objects. “The goal is to observe these very early galaxies and get an idea of all the processes that allowed these galaxies formation,” says Moseley. “Further, we hope to get a better picture of how you get from a messy universe with small irregular galaxies to the rather large spiral galaxies of today’s universe.”
The effort was forged by NASA’s Goddard Space Flight Center using micromechanical technology to manufacture the microshutter subsystem. The tiny shutters, or “trapdoors,” were made of silicon nitride and attached to the silicon wafer by hinges, explains Murzy Jhabvala, chief engineer of Goddard Instrument Technology and Systems Division.
Moseley’s team designed the shutters to open and close in response to a magnetic field. The researchers deposited magnetic material on the surface of the shutters and placed a magnet underneath to open them. Removing the magnet allows the shutters to spring closed again. To keep them open, engineers apply a voltage–a positive voltage on the shutter itself and a negative voltage on the back wall. Metallic layers serve as electrodes on the front and back of the array.
“The positive and negative voltage keeps the shutter open, and when the magnet moves away the shutter will stay open,” says Jhabvala, “and then we let the voltage go on to those shutters we want to let close. In this way, we can specifically open or close any one or hundred of the shutters we want, known as random access addressing.”
Microshutters compete with an alternate approach called micromirrors. This approach uses an array of tiles; by tilting the tiles’ “mirrors,” light is deflected, says Jhabvala. “While this is very good technology developed for projection television systems, one of our key requirements is to completely block all light. There cannot be any leakage that will corrupt the signal. The mirrors are only deflecting the light somewhere else, leaving the possibility that light could get back into the system.” The shutters completely block it, he says.
In the next six to nine months, before the microshutters are shipped to the ESA, engineers will continue fine-tuning the device. But so far it has done well; the technology has shown that it can survive the rigors of launch into space, and it works fine in the cold temperature (−388 °F).
While it’s too soon to say if this high-end space MEMS will ever be commercialized, Moseley says the advantages of absolute control of light could have relevance to medical imaging and other imaging applications. “The capability this technology offers is great. If lots of people could get it, lots of people would want it. But to make it useful for other technical imaging applications … we need to be able to scale these up to a larger size.”
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