A nuller is an optical instrument, e.g., a beam combiner for nulling interferometry, used to block or cancel out a strong source so that fainter signals near that source can be observed. This is typically useful to detect off-axis exoplanet or exozodiacal dust by interferometrically nulling out the on-axis parent star. The VLT interferometer’s visiting instrument Asgard's submodule NOTT (Nulling Observations of exoplaneTs and dusT) contains such a nuller to combine four telescope signals to perform nulling interferometry.
Nulling interferometry Nulling interferometry evolved from traditional stellar interferometry. The key difference is that it induces a 180-degree phase shift, for example, in one beam of a two-telescope interferometer. This creates destructive interference on the optical axis (where the star is) and constructive interference for off-axis objects (like planets). A technique initially proposed by Australian-American astronomer Ronald N. Bracewell in 1978 . A different technique is called a coronagraph, using a physical obstacle to block the unwanted signals.
See also Coronagraph Interferometry
References
External links Interferometric Nulling at TNO Nulling interferometer basics Tutorial on nulling interferometry Progress Toward Astronomical Nulling (Criteria governing efficacy of nulling interferometry), Chapter 10 of the Terrestrial Planet Finder (TPF): A NASA Origins Program to Search for Habitable Planets - JPL Publication 99-003, May 1999 Interactive "virtual" interferometer