May 5, 2008

Look Ma, No Lens! Just a Sheet of Metal for Planet-hunting!

Fresnel ImagerA proposed telescope would focus light primarily with a patterned sheet of metal rather than a large mirror or lens. The telescope would have amazingly sharp vision and could spot Earth-size planets around other stars.
The technique takes advantage of light's diffraction, which enables light to be focused into an image simply by passing it through a certain pattern of holes carved in an opaque sheet. Such patterned sheets have long been used for focusing laser beams, but have so far not been used for astronomy, much more for Exoplanetology. These patterned metal plates are called Fresnel zone plates, after the French physicist Augustin-Jean Fresnel, who studied diffraction in the 1800s.
Because it relies on a foil sheet rather than a massive mirror, it could be much more lightweight, and therefore less expensive to launch, than a traditional telescope.
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 just 10% or so of the light. It can also observe in the ultraviolet and infrared, in addition to visible light.
The imager can take very detailed images with high contrast, which is great for being able to see a very faint object in the close vicinity of a bright one, making it perfect for obtaining images of exoplanetary systems. Such images have so far been very difficult to make because exoplanets are so faint they get lost in their host stars' glare.
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.
Now don't get excited yet because there still a lot of challenges for this approach, such as how to unfurl the plate during deployment in space. Oh and by the way, the fresnel plate may still require a secondary mirror called a fresnel lens to correct some errors, har har. The blog title was just a trick!


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