Photon Sieve to Fly on Cubesat
Technology Demonstrated for the First Time in Solar Observation
Some of the more intriguing components of the sun’s chromosphere can’t be resolved with existing spacecraft or ground-based telescopes. However, a new imaging technology — the so-called photon sieve — promises to bring these structures to light and help scientists better understand this irregular layer above the photosphere that contributes to the formation of solar flares and coronal mass ejections.
Goddard scientists Adrian Daw and Douglas Rabin are collaborating with researchers at the U.S. Air Force Academy and other Air Force-affiliated organizations to build a small solar observatory equipped with an eight-inch (20-centimer) diffractive optic called the photon sieve. Its flight on a three-unit Cubesat in 2014 — the Air Force-sponsored FalconSat-7 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
This version of the photon sieve was used last summer to capture the first images of the sun.
“We’ve studied the sun’s corona for years and it’s complicated. But the chromosphere, which can be seen as a thin pink layer during a total solar eclipse, is even harder to understand,” Daw said. “Things are happening there at spatial scales we can’t currently resolve with existing space- or ground-based telescopes.”
Although a large observatory comparable in size to the Hubble Space Telescope could resolve magnetic flux tubes and filamentary plasma within coronal loops, the costs to build a conventional large-aperture solar telescope is cost prohibitive, Daw said. “The photon sieve could help us overcome this obstacle and help us provide a game-changing technology for high-resolution imaging in space,” he said.
A Variant of Fresnel Zone Plates
The technology that could help bring these details to light is a variant of the Fresnel zone plate, which focuses light through diffraction rather than refraction or reflection. These devices consist of a set of alternating transparent and opaque concentric circular rings. Light hitting the plate diffracts around the opaque zones, which are precisely spaced so that the diffracted light interferes at the desired focus to create an image taken by a camera.
The sieve operates largely the same. However, the rings are dotted with millions of holes, whose sizes and positions are 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.
But perhaps the most significant advantage is that the lightweight, easily rolled and deployed film need not be pulled to a perfect optical flatness like more 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 its invention more than a decade ago by Lutz Kipp, a professor at Kiel University in Germany, researchers at the U.S. Air Force Academy’s (USAFA) 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, 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 said. “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.”
Goddard’s Contribution to FalconSat-7
Since joining the effort, which also involves the Air Force Research Laboratory and the Air Force Institute of Technology, Goddard has contributed in two significant ways.
Goddard engineer Craig Stevens, who is a member of Daw’s team, used Internal Research and Development (IRAD) and Science and Engineering Collaboration Program funding to thoroughly analyze requirements for deploying the sieve and keeping the membrane relatively flat once the Cubesat reaches its 280-mile (450-kilometer) orbit. Based on that analysis, the team is confident that a dual-hexapod deployment mechanism, equipped with lanyards and pantographs, is the best approach, Daw said.
Daw’s team also used IRAD funds to design and construct a ground-based breadboard imaging system to test a glass version of the sieve. “Using this system, we took the first-ever solar images using a photon sieve,” Daw said. “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, he added.
“These two IRAD accomplishments — a ground-based demonstration of the photon-sieve technology and the analysis of the deployment system — are major advances for deployable membrane optics,” Daw said.
He and his team also are investigating ways to extend the sieve’s wavelength range to the extreme ultraviolet, which is most interesting to solar physicists. “The corona emits most of its light in the extreme ultraviolet,” Daw said. “These developments and a successful FalconSat-7 demonstration will position us nicely for capturing a solar imaging mission in the future. This technology has lots of applications for heliophysics.”
The Office of the Chief Technologist is involved in a variety of projects, missions, and technologies.