Coronagraphs

Originally invented to study the Sun, a coronagraph is a telescope designed to block light coming from the solar disk, in order to see the extremely faint emission from the region around the Sun, called the corona. It was invented in 1930 by B. Lyot to study the Sun's corona at times other than during a solar eclipse. The coronagraph, at its simplest, is an occulting disk in the focal plane of a telescope or out in front of the entrance aperture that blocks out the image of the solar disk, and various other features to reduce stray light so that the corona surrounding the occulting disk can be studied.

However, this technology is now being refined and adapted for the purpose of studying the region around distant stars in search of planets themselves or spectral evidence of planets. One challenge with this approach lies in the diffraction of light around the edges of the occulting shape, which detracts greatly from the potential angular resolution of the image.

The diffraction pattern of a simple round telescope, for example, is a series of concentric rings with a bright central spot. Blocking the light from a star in order to see an orbiting planet requires suppressing the first several bright rings without blocking out the planet. By using a different shape, the diffraction pattern can be controlled so that the starlight is much dimmer closer to the center in some areas, and brighter in others. The telescope can be rotated about its line-of-sight so that the planet image passes in an out of the regions where the starlight is dim.

Managing this diffraction pattern isn't too difficult -- there are a number of options available to accomplish this. So, the technologies under study include various tricks to block out as much of the starlight as possible, while managing the diffraction pattern such that the planet can be seen peeping out from beyond the diffraction bands.

Other proposed solutions for dealing with scattered light within the telescope include novel-shaped apertures, odd-shaped pupils, pupil masks to suppress some of the diffraction, and deformable mirrors.

To appreciate the difficulty the phenomenon of diffraction presents to the development of a coronagraph technology for studying other solar systems, see A closer look at diffraction .

Another possibility is to combine techniques of coronagraphy with interferometry. A coronagraph could also incorporate a spectrometer, so that chemical signs of life could be sought within the light reflected from a planet.

Interferometers and Nulling

An alternative way to get a picture of a distant planet is to replace one large mirror with a number of smaller mirrors and combining their light in a process called interferometry.

Using optical interferometers to study distant planets would allow for smaller mirrors, which can obtain a resolution equal to a single telescope as big as the largest separation between the individual telescopes.

To get enough of this information to build up a good picture, the interferometer must rotate around to different relative positions and repeat the "exposures." As well as taking a picture, an interferometer can obtain spectra of the targets it is looking at.

Interferometers provide extremely good angular resolution. That means they are very good at sorting out which light waves come from which part of the star system. Additionally, an interferometer can be "tuned" so that the light coming from the exact center in the field of view (where the star is) will be blanked out or nulled, while the light from any other area will be viewed normally.