Stellar coronagraph with phase mask

The detection of faint light sources very close to a bright star is primarily limited by light scattered by the Earth's atmosphere. This source of scattered light can now be reduced by means of adaptive optics, or totally eliminated by using a telescope in space. Then diffraction by the telescope aperture becomes the primary source of scattered light. Whereas a classical Lyot coronagraph can reduce the amount of light diffracted away from the star, it becomes inefficient very close to the star. Instead of forming the stellar image on an opaque mask, here it is proposed to use a small phase plate which produces a 180 degrees phase shift on the core of the stellar image. Light diffracted outside the core is then eliminated by destructive interference. Applied to the Hubble Space Telescope, the technique would easily allow detection of a stellar companion 0.'' 3 away from a star and at least 8 mag fainter.