Nulling interferometry

Nulling interferometry is a type of interferometry in which two or more signals are mixed to produce observational regions in which the incoming signals cancel themselves out. This creates a set of virtual "blind spots" which prevent unwanted signals from those areas from interfering with other, possibly much weaker signals that are nearby.

This technique is used by SIM, and is being considered for use by the Terrestrial Planet Finder, both NASA missions.

This technique is similar to a coronagraph but uses multiple sources rather than a physical obstacle to block out the unwanted signals.