Calibrating an Interferometric Null
  One of the biggest challenges associated with a nulling interferometer-based approach to detecting extra-solar Earth-like planets comes from the extremely stringent requirements of pathlength, polarization and amplitude matching in the interferometer. To the extent that the light from multiple apertures are not matched in these properties, light will leak through the nuller and confuse the search for a planetary signal. Here we explore the possibility of using the coherence properties of the starlight to separate contributions from the planet and nuller leakage. We find that straightforward modifications to the optical layout of a nulling interferometer will allow one to measure and correct for the leakage to a high degree of precision. This nulling calibration relaxes the field matching requirements substantially, and should consequently simplify the instrument design. 
Benjamin F. Lane (MIT), Matthew W. Muterspaugh (Caltech), Michael Shao   (JPL)