Photon sieves make super-cheap space telescopes

FLEXIBLE plastic telescopes launched from microsatellites could serve as quick replacements for space observatories taken out by solar flares, or spy satellites downed by military action.

The telescopes, which are being developed by Geoff Andersen and colleagues at the US Air Force Academy in Colorado Springs, rely on an imaging device called a photon sieve. Traditional telescopes use lenses or mirrors to focus light by refraction or reflection, but the photon sieve uses diffraction instead. The sieve is an ultra-thin plastic disc perforated by millions of microscopic holes, each of which bends light at different angles to create a focal point.

Less light reaches the focal point compared with traditional lenses or mirrors, making it hard to image dim objects, and the device can only take black-and-white pictures. But the sieve is cheap, lightweight and easy to manufacture at large sizes. It can also be tightly folded and unfurled without being damaged. "You can't do that with mirrors or lenses," says Andersen, who hopes to launch a device into orbit in 2014.

The planned 20-centimetre-diameter telescope will be scrunched up inside a CubeSat, a microsatellite just 10 ? 10 ? 30 cm, designed for cheaply carrying small payloads. Andersen's team aims to take pictures of the sun to prove that the concept works. A similar device could also help the search for Earth-like planets, Andersen says, though such images would require a big telescope, and would likely be just a few pixels wide.

The US Defense Advanced Research Projects Agency is also interested in using the concept to build a 20-metre version of the photon sieve for imaging objects on the ground at sub-metre resolutions.

Andersen says the design is partly a response to China's demonstration in 2007 of an anti-satellite missile. "That showed a billion-dollar national asset could be shot down at any time," he says. He will present the research at the Defense, Security and Sensing conference in Baltimore, Maryland, next month.

Marek Kukula of the Royal Observatory Greenwich in London says that while the devices won't replace the likes of the Hubble or James Webb space telescopes, a "cheap and cheerful" alternative to smaller telescopes would have many applications.

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