A nuller is an optical tool used to block a strong source so that fainter signals near that 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 the star's image. As a result, the faint light from a ring of dust orbiting the star 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.^[1][2] This technique was considered for use by both the Terrestrial Planet Finder (a canceled NASA mission) and Darwin (a canceled ESA mission). It is being used on the Keck Interferometer.

A different technique is called a coronagraph, using a physical obstacle to block the unwanted signals.

There has been a nuller built by the Jet Propulsion Laboratory that has flown on a NASA sounding rocket twice, once in 2011 and a second time in 2015. There is also a laboratory nuller at NASA Goddard Space Flight Center known as the Visible Nulling Coronagraph (VNC) that is actively conducting experiments.^{[citation needed]}

• Coronagraph
• Interferometry