Comparison of high dynamic range near-neighbor detection approaches for TPF

While observations of stellar Doppler shifts have recently uncovered a population of massive planets orbiting nearby stars, the direct detection of radiation from Earth-like planets is still well beyond reach, because of the high contrast ratios and small angular separations involved. However, a number of high-contrast near-neighbor detection techniques have the potential to enable the direct detection of planets as faint as the Earth from space-borne platforms. Several concepts, ranging from large-aperture visible wavelength coronagraphs to multi-spacecraft infrared interferometers, are currently being considered in the context of NASA's Terrestrial Planet Finder program. Because of their demanding requirements, identification of the most feasible near-term approach calls for careful consideration of the possibilities. This paper compares the optical requirements inherent in two of the primary candidate architectures: nulling interferometry and coronagraphy. At a given wavelength, both approaches share similarly challenging wavefront phasing requirements, but nulling has the advantage that it is sensitive closer to the optical axis. The phasing and dynamic range requirements are eased significantly in the thermal infrared as compared to the visible, but longer baselines and cryogenic cooling are then required. Thus, all of the options are rather complex.