Square telescopes will show up planets

17 Jun 2002

Changing the shape of a telescope's aperture can "filter out" diffracted starlight that obscures the view of Earth-like planets

Round telescopes could be a thing of the past, at least as far as searching for Earth-like planets goes. NASA is looking at putting telescopes into space that have square apertures.

Researchers at the Harvard-Smithsonian Center for Astrophysics say that problems caused by diffraction can actually be turned into an advantage and open the possibility of seeing Earth-like planets that are five times closer to their parent star than can be seen using ordinary telescopes.

When light passes through an aperture some of it is diffracted, which creates an image with a blur round it that, on star images, swamps the light from nearby planets. A square aperture causes all of the diffracted light to fall along two perpendicular axes that cross at the point of the image. Rotating the aperture should allow the image of a planet to fall in a dark zone where it is not swamped by diffracted light.

Astronomers have always been put off from using square apertures because of the fuzzy, grid effects that are produced. The sharper the edges of the square, the fuzzier the image. However, the reverse also holds true: making the edges of the aperture fuzzy produces sharper images. Computer simulations carried out by the researchers - to be reported in The Astrophysical Journal - show that a planet one-billionth the brightness of its neighbouring star could be brought into view.