Improvement in the rejection rate of a nulling interferometer by spatial filtering

We propose applying the techniques of spatial filtering to the concept of interferometric coronography. In such a system, provided that the object being studied is not resolved by the individual apertures of the interferometric array, the beams can be considered as coherent or, more exactly, single mode. Hence spatial filtering allows one to cleanse the beams of imperfections generated by defects on the optical components of the interferometer and thus to obtain very high rejection rates in the destructive output of the interferometer (coronographic output) for an on-axis star. Numerical simulations show that the very stringent constraints on the optical quality of a space IR interferometer aimed at detecting extrasolar planets can be relaxed to values achievable with current technology. In particular, we show that the difficulties induced by dust scattering, small micrometeorite impacts on the primary mirror, and high-frequency ripples of polishing residuals can be eliminated by simple pinhole spatial filtering. The effects, however, will be less dramatic on large-scale defects such as coating defects and pointing errors in the telescopes.