Nulling interferometer

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A nulling interferometer is an astronomical instrument used to observe weak sources that are very close to bright sources.

Nulling interferometry

Interferometry is used in astronomy to achieve a higher spatial resolution by combining the light from several telescopes. The technology is based on the wave nature of light. It was developed in radio interferometry and is now also used in the infrared range and in visible light. Normally, the light from the telescope is superimposed in such a way that the signals for the target object are amplified, ie they interfere constructively .

For the observation of faint objects around bright stars, the modified concept of the nulling interferometer is of interest. Here the light from the telescope is superimposed in such a way that the signals for the target object (the star) cancel each other out ('zeros'). In the simplest case of two telescopes, this applies when the signals from both telescopes for the target object are phase-shifted by exactly half a wavelength . The extinction does not apply to faint objects nearby, whose light has traveled a different path and has a different displacement. Such objects are thus visible with better contrast.

application

It is hoped that the young technology of nulling interferometry will be able to discover faint objects near stars in the future. These objects include exoplanets , dust disks, and asteroid belts . These objects cannot be filtered out of the glaring light of their central stars in the optical spectral range with today's detectors (except in exceptional cases with faint stars or with a large distance between star and planet).

The coronograph has a similar effect to a nulling interferometer, but it is based on a completely different principle. A coronograph blocks the light of a star via an opaque material in the beam path of a telescope and thus makes the faint corona of our sun visible, for example (hence the name coronograph).

History and future

The concept of nulling interferometry was proposed in 1979 by Ronald N. Bracewell and Robert H. MacPhie. In 1997 a demonstration experiment was successful at the Multiple Mirror Telescope on Mount Hopkins (Arizona) . Today, nulling interferometry is the basis of many experiments and satellites that are under construction and planned for the search for exoplanets, such as B. GENIE (a test experiment under construction for the Very Large Telescope ), the Large Binocular Telescope in Arizona , which will go into operation in autumn 2005, and some space telescopes such as NASA's Terrestrial Planet Finder (TPF) (which has since been canceled for cost reasons) and that too deleted Darwin from ESA , which should start looking for earth-like planets from 2015.

literature

  • RN Bracewell, RH MacPhie: Searching for nonsolar planets , Icarus 38, 136 (1979)