= Coronagraph
{wiki}
A <coronagraph> is an <optical instrument> that suppresses a bright source's <light> before detection while transmitting some <light> from nearby directions. Its purpose is to reveal faint structure buried in the bright source's <point spread function>, as in <exoplanet direct imaging>. A <Lyot coronagraph> combines a <coronagraphic focal plane mask> with a <Lyot stop> in a reimaged <optical pupil>; blocking the central image alone does not remove its <diffraction> rings. https://subarutelescope.org/staff/guyon/15teaching.web/03AstrOptics2014.web/wdir.web/AstrOpt2014_20highcontrast.pdf[Olivier Guyon, Coronagraph concepts & systems, 2014, PDF pages 3 and 6]{ref}.
\Image[/coronagraph-lyot-optical-planes.png]
{title=The four principal planes of a <Lyot coronagraph>: a <focal plane> mask and a reimaged <optical pupil> stop select different parts of the <wave amplitude> distribution. Qualitative schematic, with illustrative sizes and brightness.}
The original motivation was the <solar corona>: hiding the bright <photosphere> would allow longer, repeated observations of faint surrounding <light>. In his 1930 paper, Lyot explained that short total eclipses restricted observing time and prevented some long exposures for <spectral lines>. The attraction is an instrument that makes the faint neighborhood accessible on demand. https://www.persee.fr/doc/bastr_0245-9760_1930_num_6_1_14031[Bernard Lyot, La couronne solaire étudiée en dehors des éclipses, 1930, p. 305]{ref}. The <Sun> is an extended source; the point-source calculation below illustrates imaging of an unresolved <star> and is not a quantitative model of the <solar corona>.
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