Analysis and design of an occulter for exoplanet missions.

摘要

The presence of a bright spot, known as Poisson Spot, in the center of the geometrical shadow of an opaque circular disk illuminated by light has been known for close to two hundred years. In astronomy, detection of exosolar planets requires a reduction of the bright central starlight by up to 10 orders of magnitude so that the faint reflected light from an orbiting planet could be detected by a telescope. In this study we investigate the degree to which the near axis intensity of a normally incident electromagnetic wave within the visible spectrum can be reduced by shadowing with a suitably designed occulter. Although an existing proposed solution employing a petal style configuration appears to meet the required intensity reduction, this performance can only be achieved with unreasonably small radii of curvature at the petal tips. We examine two alternative solutions. One is a circular disk of neutral density filter with an optimized transparency profile the other a hybrid occulter with partial transparent petals. Unlike in the totally opaque petal style occulter, the tip radii of curvature of the hybrid occulter can be chosen with reasonable radii of curvature and compatible with the constraints of physical optics. While both the circular and the hybrid occulter yield an average intensity reduction of at least 10 orders of magnitude, the hybrid occulter has the advantage that only a relatively small fraction of its area needs to be semitransparent. This should significantly simplify the construction of an experimental model.