A Fresnel imager is a proposed ultra-lightweight design for a space telescope that uses a

Fresnel Augustin-Jean Fresnel (10 May 1788 – 14 July 1827) was a French civil engineer and physicist whose research in optics led to the almost unanimous acceptance of the wave theory of light, excluding any remnant of Newton's corpuscular the ...

array as primary optics instead of a typical lens. It focuses light with a thin opaque foil sheet punched with specially shaped holes, thus focusing light on a certain point by using the phenomenon of

diffraction Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...

. Such patterned sheets, called Fresnel

zone plate A zone plate is a device used to Focus (optics), focus light or other things exhibiting wave character.G. W. Webb, I. V. Minin and O. V. Minin, “Variable Reference Phase in Diffractive Antennas”, ''IEEE Antennas and Propagation Magazine'', ...

s, have long been used for focusing laser beams, but have so far not been used for astronomy. No optical material is involved in the focusing process as in traditional

telescopes A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observ ...

. Rather, the light collected by the Fresnel array is concentrated on smaller classical optics (e.g. 1/20 of the array size), to form a final image. The long

focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative foc ...

s of the Fresnel imager (a few kilometers) require operation by two-vessel formation flying in space at the L2 Sun-Earth Lagrangian point. In this two-spacecraft formation-flying instrument, one spacecraft holds the focussing element: the Fresnel interferometric array; the other spacecraft holds the field optics, focal instrumentation, and detectors.

Advantages

* A Fresnel imager with a sheet of a given size has vision just as sharp as a traditional telescope with a mirror of the same size, though it collects about 10% of the light. * The use of vacuum for the individual subapertures eliminates phase defects and spectral limitations, which would result from the use of a transparent or reflective material. * It can observe in the ultraviolet and infrared, in addition to visible light. * It achieves images of high contrast, enabling observation of a very faint object in the close vicinity of a bright one. * Since it is constructed using foil instead of mirrors, it is expected to be more lightweight, and therefore less expensive to launch, than a traditional telescope. * A 30-metre Fresnel imager would be powerful enough to see Earth-sized planets within 30 light years of Earth, and measure the planets' light spectrum to look for signs of life, such as atmospheric oxygen. The Fresnel imager could also measure the properties of very young galaxies in the distant universe and take detailed images of objects in the

Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

Development

The concept has been successfully tested in the visible, and awaits testing in the UV. An international interest group is being formed, with specialists of the different science cases. A proposal for a 2025-2030 mission has been submitted to ESA Cosmic Vision call. In 2008 Laurent Koechlin of the

Observatoire Midi-Pyrénées The Observatoire Midi-Pyrénées (OMP) is an astronomical observatory part of Toulouse III - Paul Sabatier University, France. It federates the laboratories of the (astronomy, astrophysics, cosmology), planetary sciences and the environmental s ...

in Toulouse, France, and his team planned to construct a small ground-based Fresnel imager telescope by attaching a 20-centimetre patterned sheet to a telescope mount. Koechlin and his team completed the ground-based prototype in 2012. It uses a piece of copper foil 20 cm square with 696 concentric rings as the zone plate. Its focal length is 18 metres. They were able to resolve the moons of Mars from the parent planet with it.

Advantages

Development

See also

References

Further reading