15.1 Introduction

Many interesting astronomical objects within the FOV of a telescope are characterized by great differences in brightness. The coronagraph is an optical system configuration designed to measure very faint objects in the presence of very bright objects by blocking unwanted radiation using an innovative technique. Some examples of the applications of coronagraphy are the characterization of exoplanets, the evolution of binary star systems, and studies of the faint gas and dust that surrounds some bright celestial objects such as stars. Ground-based telescopes at the threshold of detection measure the very faint solar corona, which is less than 10⁻⁷ times the brightness of the solar photosphere, the sun's apparent surface. The white-dwarf companion of Sirius (α CMa), which is 10⁻⁴ times as bright as Sirius with a separation of only 4 arcsec was observed. These measurements were accomplished without the aid of adaptive optics. Several investigators have analyzed the possibility of combining adaptive optics with ground-based coronagraphs to improve the probability of imaging an exoplanet from the earth's surface, beneath the turbulent atmosphere. Groundbased telescopes are limited by airglow in the atmosphere and the forwardscattering properties of the earth's atmosphere. Small dust particles and aerosols in the atmosphere cause narrow-angle forward scattering.

Optical telescopes in orbit about the earth are above the airglow and atmospheric scattered light. In space (within the solar system), the limits to unwanted radiation are the zodiacal light and the gegenschein within the solar system, and the scattered light within the telescope system.