Wiktionary
n. Any object, natural or man-made, that blocks the light of an object from an observer, typically used in reference to astronomical events.
Usage examples of "occulter".
In this column I will focus on only one of them, the diffraction-suppressed occulter proposed by Professor Webster Cash of the University of Colorado.
In other words, the occulter blocks the light from the star but not from its planets.
The distance from occulter to telescope is typically around 104 kilometers or more, large enough that the occulter must be located in space and must be carefully navigated into a position where the planets of a candidate star might become visible in the star-shadow over some period of time.
However, there is a basic wave-optics problem with the occulter scheme.
In realistic situations, this diffracted starlight is much stronger than the light from planets around the star, and because of diffraction effects a circular disk is not useful as an occulter in searching for extrasolar planets.
This is done by the smoothing or tapering of the sharp edge of the occulter in such a way that the waves from different regions tend to cancel and diffraction is suppressed.
However, such a gray-edge occulter would have many problems in space-based applications.
The zodiacal light is reduced by about a factor of 10 by the occulter, but it is not eliminated.
However, Cash has shown with simulations that, even with the halo of zodiacal light, at a distance of 23 light years the occulter and the JWST could observe Venus, Earth, Mars, and Jupiter as bright spots in an image taken of the Solar System.
Pinhole penetrations of the black object by micrometeorites should not be much of a problem, since the occulter could be designed to self-seal around small penetrations.
Moreover, even if the occulter developed many pinholes, the diffraction effects would spread the penetrating light over an area much larger than the images of interest, reducing its effect.