Fresnel Imager - AbsoluteAstronomy.com

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

Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

. Such patterned sheets, called Fresnel zone plate

Zone plate

A zone plate is a device used to focus light or other things exhibiting wave character. Unlike lenses or curved mirrors however, zone plates use diffraction instead of refraction or reflection. Based on analysis by Augustin-Jean Fresnel, they are sometimes called Fresnel zone plates in his honor...

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

The long focal lengths 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's 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 our own solar system.

Development

The concept has been successfully tested in the visible, and awaits testing in the UV

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

. 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

Cosmic Vision

The Cosmic Vision is the paraphrasing name given to the roadmap for scientific space based missions of the European Space Agency in the time frame between 2015 and 2025....

call.
Laurent Koechlin of the Observatoire Midi-Pyrénées in Toulouse, France, and his team plan to construct a small ground-based Fresnel imager telescope by attaching a 20-centimetre patterned sheet to a telescope mount.

Advantages

Development

See also

Further reading