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.
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. 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.
See also 
- Bracewell, Ronald N. (1978), "Detecting nonsolar planets by spinning infrared interferometer", Nature (1978-08-24) 274 (5673): 780–781, Bibcode:1978Natur.274..780B, doi:10.1038/274780a0, ISSN 0028-0836 More than one of
- Bracewell, Ronald N.; MacPhie, Robert H. (1979), "Searching for nonsolar planets", Icarus (1979-04-01) 38 (1): 136–147, Bibcode:1979Icar...38..136B, doi:10.1016/0019-1035(79)90093-9, ISSN 0019-1035 More than one of
- Interferometric Nulling at TNO
- Nulling interferometer basics
- Tutorial on nulling interferometry
- Criteria governing efficacy of nulling interferometry
- Interactive "virtual" interferometer
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