A Coronagraph with a Band-limited Mask for Finding Terrestrial Planets

Several recent designs for planet-finding telescopes use coronagraphs operating at visible wavelengths to suppress starlight along the telescope's optical axis while transmitting any off-axis light from circumstellar material. We describe a class of graded coronagraphic image masks that can, in principle, provide perfect elimination of on-axis light, while simultaneously maximizing the Lyot stop throughput and angular resolution. These "band-limited" masks operate on the intensity of light in the image plane, not the phase. They can work with almost any entrance pupil shape, provided that the entrance pupil transmissivity is uniform, and can be combined with an apodized Lyot stop to reduce the sensitivity of the coronagraph to imperfections in the image mask. We discuss some practical limitations on the dynamic range of coronagraphs in the context of a space-based terrestrial-planet finder telescope and emphasize that fundamentally, the optical problem of imaging planets around nearby stars is a matter of precision fabrication and control, not Fraunhofer diffraction theory.