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April 03, 2012

Photon Sieve will make Space Telescopes one thousand times lighter for the same sized collection area

FalconSAT-7 is a 3U CubeSat satellite measuring just 30cmx10cmx10cm. It is a DARPA funded project that is led by Geoff Andersen and colleagues at the US Air Force Academy. They hope to launch a device into orbit in 2014. While not optimized for ground observation, such a telescope would have a 1.8 meter resolution at an
orbital altitude of 450km. This technology as a potential game-changer in space surveillance for both ultra-small and ultra-large applications. With the use of polyimide membranes they are aiming at areal densities of 0.25 kg/m2 – a game-changing 3 orders of magnitude improvement above current state-of-the-art in space telescopes.

The payload is Peregrine: the world’s first space-based membrane telescope. The program goals are:
• Deploy a rigid structure supporting a 0.2m membrane photon sieve
• Image the Sun at the H-alpha wavelength of 656.3nm

Solid Works picture of Peregrine, a 0.2m photon sieve deployed from a 3U CubeSat.




A photon sieve is a novel optical element consisting of a flat opaque sheet with millions of tiny holes. Light passing through these holes is focused in a similar manner to a lens or a mirror. Photon sieves have several key advantages over those more conventional optics:
• Focusing can be achieved from a flat, thin sheet that can be unfurled from a very compact, lightweight package
• Surface quality tolerances are orders of magnitude more relaxed
• The fabrication costs are much lower The trade-offs include:
• Lower efficiency / loss of light
• Narrow bandwidth giving what are essentially grayscale images

Clockwise from top left: A 4-inch photon sieve lit by laser light. The focal spot produced. A magnified image of the central 25mm. An image of a resolution chart produced by the sieve. An interferogram of the wavefront that indicates perfect focusing capability.


The photon sieve will have the following design parameters:
• 200mm diameter, 400mm focal length, 656.3nm wavelength
• 2.5 billion holes ranging in size from 2-277 microns
• 50% fill factor, 30% focusing efficiency The telescope has a relatively simple design due to space constraints and has:
• 4 μrad resolution which equates to 600 km at Sun surface
• ~0.1 degree field of view (about a 1/5th of the Sun’s disk)
• 1 Ångstrom spectral bandwidth A schematic of the secondary optics is shown below.


The primary, support structure, two cameras, secondary imaging elements and imaging electronics are all configured to fit within one half of the 3U satellite, with the other half occupied by communications, power, attitude control and miscellaneous avionics.

As well as having a revolutionary focusing element, Peregrine will demonstrate a novel deployable structure. The deployment mechanism consists of pantographs and lanyards designed to deploy the primary from a stowed configuration to a flat sheet under tension. A series of images showing the deployment process is shown below.

FalconSAT-7: A Photon Sieve Solar Telescope

Telescope technology has only incrementally improved in areal mass since the beginning of space-based imagery.
For example, the Hubble Space Telescope has a mirror with 180 kg/m2 while the James Webb Space Telescope has
reduced this to just 25 kg/m2 over a quarter of a century later. Not only is size an issue but the cost of fabricating
surfaces to the high degree of precision results in telescope costs scaling roughly as the diameter to the power of
1.75. Added to this there is still the issue of packing large monolithic structures into limited launch vehicle volumes.

The membrane photon sieve has several advantages over that of a primary mirror or lens used in traditional imaging
systems. First, the material is made on a flexible membrane that can be folded. This allows for deployed apertures
with a diameter larger than the satellite bus. Second, the PS surface requirement is around two orders of magnitude
less stringent than that of traditional optical surfaces. Third, the PS is extremely lightweight. With the use of
polyimide membranes we are aiming at areal densities of 0.25 kg/m2 – a game-changing 3 orders of magnitude
improvement above current state-of-the-art
. This comes along with similar savings in fabrication a materials cost.

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