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

Several recent designs for planet-finding telescopes use coronagraphsoperating at visible wavelengths to suppress starlight along the telescope'soptical 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 maximizingthe Lyot stop throughput and angular resolution. These ``band-limited'' masksoperate on the intensity of light in the image plane, not the phase. They canwork with almost any entrance pupil shape, provided that the entrance pupiltransmissivity is uniform, and can be combined with an apodized Lyot stop toreduce the sensitivity of the coronagraph to imperfections in the image mask.We discuss some practical limitations on the dynamic range of coronagraphs inthe context of a space-based terrestrial planet finder (TPF) telescope, andemphasize that fundamentally, the optical problem of imaging planets aroundnearby stars is a matter of precision fabrication and control, not Fraunhoferdiffraction theory.Comment: 24 pages, including 6 figures. To appear in the Astrophysical Journal, May 200