Greenbelt, MD--Adrian Daw and Douglas Rabin, scientists at NASA's Goddard Space Flight Center, are collaborating with researchers at the U.S. Air Force Academy (USAFA; El Paso County, CO) and other Air Force-affiliated organizations to build a small space solar observatory equipped with a "photon sieve," a 200-mm-diameter device that is a form of diffractive optic. A version of this technology was successfully demonstrated in a ground test, paving the way for its flight on a miniature Cubesat satellite in 2014—the Air Force-sponsored FalconSat-7 mission. That mission will demonstrate the practicality of deploying this emerging technology in space and possibly paving the way for a larger heliophysics mission in the future.
A diffractive optic formed entirely of holes
In a photon sieve, ring-shaped diffractive zones rings are dotted with millions of holes with sizes and positions configured so that the light diffracts to a desired focus. As a result of its design, the sieve can be patterned on a flat surface and can be easily scaled up in size—particularly if constructed of a polyimide film similar to the ubiquitous Kapton, which spacecraft and instrument developers commonly use because it can withstand extreme temperatures and vibration—for example, it is used for the sunshades in the James Webb Space Telescope (JWST).
Perhaps its most significant advantage is that the lightweight, easily rolled, and deployed film need not be pulled to a perfect optical flatness like traditional mirrors. In fact, surface requirements for traditional mirrors are 100 times more stringent, making the photon sieve ideal as a quick-turnaround space-based optic. This appeals to the Air Force. In the event of a catastrophic loss of its current intelligence, surveillance, and reconnaissance satellites, the military would need a simple replacement system easily deployed from a small, inexpensive satellite, like a Cubesat.
Since the invention of the photon sieve more than a decade ago by Lutz Kipp, a professor at Kiel University in Germany, researchers at the USAFA's Laser and Optics Research Center have experimented with different materials for making the sieve. USAFA's Geoff Andersen, Michael Dearborn, and Geoff McHarg initially experimented with chrome-coated quartz or glass, later focusing their efforts on lightweight polyimide films or membranes. In laboratory testing, these sieves showed great promise for narrow and broadband imaging in visible wavelength bands, particularly in the H-alpha wavelength band ideal for detecting structure within the solar chromosphere.
What they lacked, however, was expertise in solar physics and some of the analytical tools needed to evaluate the sieve's deployment mechanisms. "They were looking for the best way to demonstrate their technology," Daw says. "It's easier to test imaging technologies with a really bright source, like the sun. They contacted us to see if we wanted to collaborate. Of course we did. The collaboration is proving to be of great mutual benefit to both organizations."
Since joining the effort, which also involves the Air Force Research Laboratory and the Air Force Institute of Technology, both located at the Wright-Patterson Air Force Base in Ohio, NASA Goddard has analyzed requirements for deploying the sieve and keeping the membrane relatively flat once the Cubesat reaches its 450 km orbit.
Daw's team also designed and constructed a ground-based imaging system to test a glass version of the sieve. "Using this system, we took the first-ever solar images using such a device," Daw says. "In fact, these are the first images of any astronomical object using a photon sieve." The next step is carrying out ground-based tests of the membrane sieve. Daw and his team are also investigating ways to extend the sieve's wavelength range to the extreme ultraviolet, which is most interesting to solar physicists.
Source: http://www.nasa.gov/topics/technology/features/kitchen-optics.html
