No abstract

No abstract

ABSTRACT. 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° 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.

No abstract

We have calculated the efficiency with which starlight can be coupled into a single-mode fiber optic that is placed in the focal plane of a telescope. The calculations are performed for a wide range of seeing conditions, with and without rapid image stabilization, and for a wide range of wavelengths. The dependence of coupling efficiency on the f-ratio of the incident beam is explored. Also, we calculate the coupling efficiency as a function of displacement for a perfect Airy pattern. We have also used a computer program which simulates atmospheric wavefronts to determine the variance of instantaneous coupling efficiency as a function of seeing. In perfect conditions, the maximum efficiency at the LP(11) mode cutoff is 78% due to the mismatch of the Airy pattern and the nearly Gaussian mode of the fiber. Maximum total coupled power is attained at d/r(0) = 4 with rapid image stabilization.