'Cling film' space telescope to image Sun

Testing on a zero-g flight Testing the deployment of the membrane lens on a microgravity aeroplane flight

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A space-based telescope that uses a thin plastic membrane as a lens is being developed by the United States Air Force.

Called a photon sieve, it works by bending light through billions of tiny holes in the "cling film" style optic.

Unlike in existing traditional mirror-based telescopes, the new lens can be folded to fit into a tiny space.

A test device is due to be launched in 2015 on a small satellite, or cubesat, to capture images of the Sun.

Conventional telescopes work by using mirrors to focus light. But in a photon sieve, each tiny hole in the plastic lens causes light to diffract.

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While the size of telescope we will deploy is 20cm, we're doing that from a small volume, perhaps the size of a cup of coffee”

End Quote Dr Geoff McHarg US Air Force Academy

The holes are arranged in such a way as to create an image from the light source.

Size is everything

The idea has been developed by the US Air Force Academy, and it hopes to launch a 20cm diameter telescope inside a tiny satellite called FalconSAT-7 in 2015.

Dr Geoff McHarg is a professor of physics at the academy and has been working on the project for several years. He told BBC News that the big advantage of the new telescope was its size.

"To date, all space-based telescopes are limited by the physical size of the spacecraft they fit in. The Hubble is the size it is because it had to fit in the space shuttle," he explained.

"While the size of telescope we will deploy is 20cm, we're doing that from a small volume - perhaps the size of a cup of coffee."

Photon sieve A magnified view of a photon sieve showing concentric rings of holes

The ability to crunch up the lens inside a small satellite would mean a huge saving in costs.

Dr Geoff Andersen works on developing the optics for the new telescope at the academy's centre in Colorado Springs.

"To put a can of cola in space in terms of mass costs about $25,000. If we're looking at something that's almost three orders of magnitude lighter than existing technology, then it's about 1% of the costs right there in mass alone. That's a huge saving you can make."

Even though the images of the Sun produced by a 20cm telescope will not bring any great scientific insight, the US space agency (Nasa) is working with the Air Force on the project because it is keen on the potential.

"They are hoping that if we can show that this works, that you could in fact fly a larger telescope made out of cling film that you could basically roll up and put into a small spacecraft and then deploy it," says Dr McHarg.

"That would save enough money so you could afford to fly the mission."

Deploying the photon sieve The photon sieve is pushed out from the cubesat, unwrapped and pulled taut

While the technology has many advantages terms of size and cost, it does have some limitations, too. It is harder to image dim objects because less light reaches the focal point, says Dr Andersen.

"It's not a free lunch," he adds. "What you also lose is the ability to look at all colours at once. We're not going to be able to take a nice three-colour image. What you are essentially going to get is a nice image in black and white."

The research team has carried out a test in zero gravity to demonstrate that the telescope can be deployed from a cubesat that measures 10cm by 10cm by 30cm, about the size of a milk carton. The team will soon be carrying out so-called "shake and bake" testing - subjecting the assembled device to vibration and vacuum stresses.

If it survives, the researchers at the academy say that getting into orbit in 2015 is a realistic target. If it happens it will be the first flight in space for an optical membrane telescope.

While nothing has been put in writing, Dr McHarg is confident that the new telescope will be able to hitch a ride on an existing mission.

"Because all of that is containerised, it's a lot less hard to get us a ride than it might be otherwise; that's why we're fairly hopeful that this ride in 2015 will come about," he said.

"The big payloads go off and then a door opens and we're pushed out into space like a can of peas. We're sorta chucked overboard; we're the very last guys out of the ship."

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