Foldable, membrane-based orbital telescope could alter space vision

DARPA’s membrane-based Geosynchronous Earth Orbit telescope doesn’t use glass or mirrors for imagery

The Defense Advanced Research Projects Agency is looking to shelve the heavy glass lenses and mirrors that make up the key components of space telescopes.

The agency said this week it successfully demonstrated a ground-based prototype telescope that used lightweight polymer membrane optics to replace traditional glass mirrors as part of its Membrane Optic Imager Real-Time Exploitation (MOIRE) project.

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 DARPA says MOIRE aims to create technologies that would let future high-resolution orbital telescopes provide real-time video and images of the Earth from Geosynchronous Earth Orbit (GEO)-roughly 22,000 miles above the planet's surface. Size, weight and cost limitations have so far prevented placing large-scale imaging satellites in GEO. 

DARPA says the capability of current orbital telescopes to see wide swaths of the earth at a time has made them indispensable for key national security responsibilities such as weather forecasting, reconnaissance and disaster response.

"Even as telescope design has advanced, however, one aspect has remained constant since Galileo: using glass for lenses and mirrors, also known as optics. High-resolution imagery traditionally has required large-diameter glass mirrors, which are thick, heavy, difficult to make and expensive. As the need for higher-resolution orbital imagery expands, glass mirrors are fast approaching the point where they will be too large, heavy and costly for even the largest of today's rockets to carry to orbit," DARPA stated. MOIRE aims to change that.

In its current ground configuration, DARPA said its prototype has achieved a technological first for membrane optics by nearly doubling their efficiency, from 30 to 55%. "The improved efficiency enabled MOIRE to take the first images ever with membrane optics."

DARPA went on to note that While membranes are less efficient than glass, which is nearly 90% efficient, their much lighter weight enables creating larger lenses that more than make up the difference. The membrane is also substantially lighter than glass. Based on the prototype, a system incorporating MOIRE optics would come in at roughly one-seventh the weight of a traditional system of the same resolution and mass. As a proof of concept, the MOIRE prototype validates membrane optics as a viable technology for orbital telescopes, DARPA stated.

"Membrane optics could enable us to fit much larger, higher-resolution telescopes in smaller and lighter packages," said Lt. Col. Larry Gunn, DARPA program manager in a statement. "In that respect, we're 'breaking the glass ceiling' that traditional materials impose on optics design. We're hoping our research could also help greatly reduce overall costs and enable more timely deployment using smaller, less expensive launch vehicles."  

The key technology in membrane-based telescopes refracts light as opposed to reflecting it with mirrors and lenses, DARPA said.

"The thickness of household plastic wrap, each membrane serves as a Fresnel lens-it is etched with circular concentric grooves like microscopically thin tree rings, with the grooves hundreds of microns across at the center down to only 4 microns at the outside edge. The diffractive pattern focuses light on a sensor that the satellite translates into an image. "

If such a system were launched, it would fold up in a package roughly 20ft in diameter and upon reaching its destination orbit, the satellite would then unfold to create the full-size multi-lens optics. The envisioned diameter of 20 meters (about 68 ft) would be the largest telescope optics ever made and dwarf the glass mirrors contained in the world's most famous telescopes, DARPA stated.

From GEO, it is believed, a satellite using MOIRE optics could see approximately 40 percent of the earth's surface at once. The satellite would be able to focus on a 10 km-by-10 km area at 1-meter resolution, and provide real-time video at 1 frame per second, DARPA stated.

The MOIRE project began in 2010 and is under development by Ball Aerospace and the U.S. Air Force Academy.

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