A nuller is a tool used to block out a strong source so that fainter signals near the source can be observed. An example of a nuller is being employed on the Keck Interferometer. This causes the light from a star to destructively interfere, effectively cancelling out the star's image. The faint light from an orbiting ring of dust can then be detected. This project is part of a scientific effort to detect and observe nearby planets.
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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 weaker nearby signals.
In 1978 Australian-American astronomer Ronald N. Bracewell suggested using nulling interferometry to search for planets around other stars.[1][2] This technique is being considered for use by the Terrestrial Planet Finder, a NASA mission. Also the ESA Darwin mission is considering the use of it. It is being used on the Keck Interferometer.
This technique is similar to a coronagraph but uses multiple sources rather than a physical obstacle to block out the unwanted signals.