The problem is that these telescopes are based largely on glass lenses and mirrors. These do a splendid job with an efficiency of 90 percent, but glass has its limitations – not the least of which is its weight. Glass is heavy. On Earth, telescope mirrors are limited in size because they start to deform under their own weight. In space, its even worse because, according to DARPA, optics in satellites are rapidly reaching the point where putting a larger mirror into orbit will be beyond the power of the largest rocket.
DAPRA’s solution to this is its MOIRE program, which aims to sidestep the whole glass problem with optics made out of a lightweight polymer membrane that is not only of comparable quality to glass, but can also be folded.
MOIRE is a two-phase project aimed at creating technologies that could be used to place a future high-resolution orbital telescope in geostationary orbit for real-time video surveillance of the Earth. It uses a polymer membrane that is the thickness of household plastic wrap. This membrane doesn’t reflect or refract light like conventional mirrors and lenses. Instead, it diffracts it like a Fresnel lens by means of microscopic concentric grooves etched into the plastic. These grooves range in size from hundreds of microns wide down to only four microns.
The optical membrane is less efficient than glass, but it makes up for this in size. And since it’s lightweight and can fold up, an orbital polymer telescope’s size can be very big without weighing much. DARPA estimates that such telescopes will be one-seventh the weight of a comparable glass-mirrored version.
The idea is that the membranes would be mounted in thin metal petals that would fold up like origami to form a spacecraft about 6 m (20 ft) in diameter. Launched into geostationary orbit, the satellite would unfold a long support structure with the membrane lens at one end and a sensor suite at the other. The lens would then unfold to a diameter of 20 m (65 ft) and focus light on the sensors to form images.
According to DARPA, this would be the single largest telescope ever built – twice the size of the ground-based twin 10-m Keck telescopes. From its geostationary position, it would see about 40 percent of the Earth’s surface at a resolution of one meter while generating videos at one frame per second.
MOIRE is now in its second and final phase. With Ball Aerospace & Technologies as the primary contractor, MOIRE demonstrated a ground-based, proof-of-concept prototype. This consists of a section of a 5-m wide optic using polymer membranes to replace glass and the development of the secondary optics needed to operate the telescope. Aside from achieving the proper flatness and stability, the MOIRE telescope scored a first for membrane optics by nearly doubling their efficiency from 30 to 55 percent and creating the first images ever with membrane optics.
DARPA says that the next part of Phase 2 will be an orbital test of the optical membrane as part of the US Air Force FalconSAT-7 program.
“Membrane optics could enable us to fit much larger, higher-resolution telescopes in smaller and lighter packages,” says Lt. Col. Larry Gunn, DARPA program manager. “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.”
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