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.
In recent years, at least a dozen new types of coronagraphs have been invented that might enable us to see an Earth close to a vastly brighter star. Methods for canceling the halo of starlight scattered within the telescope have also been demonstrated, thereby dramatically relaxing the requirements for optical surface quality. In both areas, laboratory tests are currently getting close to achieving flight-quality performance. The telescope mirror itself needs to be relatively large (8m x 3.5m), but it is still within reach of current fabrication capabilities. Alternative coronagraph designs might allow the
Terrestrial Planet Finder Coronagraph mission to be accomplished with a smaller mirror.
A large visible-wavelength space telescope of the quality needed to detect habitable Earth-sized planets would also have important applications to other areas of astrophysics. In particular, it would profoundly advance the science of extragalactic observations, out to the very edge of the universe.
Topics of literally cosmic importance could be addressed, including the expansion rate of the universe, dark energy, dark matter, and the formation of the first stars after the Big Bang. With its planned wide-field camera, TPF-C could conduct many of its deep-space observations in parallel with the search and characterization of Earth-like planets. In addition, the mission would provide dedicated time for separate pointed observations in a program that would be open to the general astronomical community.
