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Lyot coronagraph

(CLC, classical Lyot coronagraph)
(original coronagraph design)

The term Lyot coronagraph or (classical Lyot coronagraph) is used for the general design of the original coronagraphs which were developed by Bernard Lyot. Coronagraphs aim to block light directly from the Sun so as to view regions near the Sun, or do the same for stars, e.g., for direct imaging of extra-solar planets, and circumstellar disks, both cases where the extreme difference in brightness makes it difficult to image the intended target. High contrast imaging (HCI) refers to such imaging.

The original coronagraph, which was designed for viewing the Sun's corona, focuses an image of the Sun and corona at a focal plane where an occulter aka occulting mask is mounted, blocking the light from the Sun's photosphere. The unblocked light proceeds, diverging until it passes through a subsequent lens that focuses the light again, and then through a stop (like a camera stop, a circular hole through which the light passes) termed a Lyot stop, which is mounted so as to reduce the ring of light from the diffraction of the Sun's light since even the small percentage of sunlight diffracted away from the center is enough to obscure the surroundings, so the Lyot stop is sized and placed so as to reduce light from the brightest ring(s) of diffracted light. The term Lyot stop refers to a stop specifically aimed at the diffraction due to the telescope's aperture (e.g., the kind that forms and Airy disk). Similar stops are used in some telescopes and coronagraphs to catch stray light from random reflections within the instrument, and sometimes they are referred to as Lyot stops.

This original general design and close variants are still used, but new coronagraph designs have been developed. There are new designs aimed at solar observation, but clearly a spur to new developments are plans to direct image exoplanets. Among the innovations being tried are the shapes of the occulters and stops, and whether they are sharp-edged or rounded, the use of phase shifting and interferometry, and of polarization as means to remove the unwanted light, and asymmetric occulters and stops. Some designs don't offer a circularly symmetrical view around the star, trading that away to improve the view close to the star in some particular direction(s), presuming the occulter will be rotated periodically during searches for planets and can be positioned appropriately to observe a known planet.


(instrument type)
Further reading:
http://lyot.org/background/coronagraphy.html
http://ircamera.as.arizona.edu/Astr_518/guyon_lecture.pdf

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