A coronagraph is a telescopic attachment designed specifically to block out the harsh, direct light from a star, so that nearby objects can be resolved without burning out the telescope's optics. The name comes from the fact that the first coronagraphs were used to image the corona of the sun.
Coronagraphs are useful for imaging coronas, near-solar comets, and even extrasolar planets.
The coronagraph was introduced in 1930 by the astronomer Bernard Lyot. Since then, missions such as NASA's SOHO, SPARTAN, and Skylab have used coronagraphs to study the sun. A coronagraph concept is currently being studied to fly on the Terrestrial Planet Finder mission.
Other example images
Here are some example images from NASA's web site.