coronagraph

A coronagraph is a feature or attachment for a telescope designed to block light from a bright source within its field of view so that other astronomical objects can be seen more clearly. Ordinarily, something very much brighter within the view prevents sensing instruments (or our eyes) from gathering data specific to other objects in view. Coronagraphs were developed to assist in viewing the Sun's corona by blocking light from the Sun's usually-visible disk; before their invention, practical observation of the corona was limited to the duration of a solar eclipse. However, coronagraph variants are now also used and designed to analogously block star light, in particular, for direct imaging of extra-solar planets. Coronagraphy is astronomy, e.g., observation, making use of coronagraphs. The more general term, high contrast imaging (HCI) is also used. The phrase inner working angle refers to the smallest angle between the planet and the star, as seen from the telescope, at which the planet can be detected.

The term occulter is also used for various devices that carry out this function and a coronagraph may be termed a type of occulter (or the portion of the coronagraph actually blocking the light may be termed its occulter). The term occulter also is used for starshades, occulting bars and for the astronomical bodies that occult during occultations.