Coronagraph

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An example image from SOHO - NASA

A coronagraph is a telescopic attachment designed specifically to block out the harsh, direct light from a star, so that nearby objects can be resolved without burning out the telescope's optics. Most coronagraphs are intended to view the corona of the Sun (which is, after all, the closest star to Earth), but similar techniques are used to find planets around other stars.

Coronagraphs are useful for imaging coronas, near-solar comets, and even extrasolar planets.

The coronagraph was introduced in 1930 by the astronomer Bernard Lyot. Since then, missions such as NASA-ESA's SOHO, SPARTAN, and Skylab have used coronagraphs to study the sun. A coronagraph concept is currently being studied to fly on the Terrestrial Planet Finder mission. On ground-based telescopes, a coronagraph can be combined with adaptive optics to search for planets around nearby stars [1].

Coronagraph instruments are studies in stray light rejection, because the total brightness from the solar corona is less than one millionth (10^-6) the brightness of the Sun.The apparent surface brightness is even fainter because, in addition to delivering less total light, the corona has a much greater apparent size than the Sun itself. In fact, Lyot is credited with having invented antireflective coatings: his description paper for the first coronagraph (JRASC 28, p. 225: "The Study of the Solar Corona Without an Eclipse" (1933)) includes a passage about using grease from the pores of his own nose to coat the optics and reduce scattered light.

The simplest possible coronagraph is a simple lens or pinhole camera behind an appropriately aligned occulting disk that blocks direct sunlight; during a solar eclipse, the Moon acts as an occulting disk and any camera in the eclipse path may be operated as a coronagraph until the eclipse is over.

Coronagraphs operating within Earth's atmosphere suffer from scattered light in the sky itself, due primarily to Rayleigh scattering of sunlight in the upper atmosphere. At view angles close to the Sun, the sky is much brighter than the background corona even at high altitude sites on clear, dry days. Ground based coronagraphs, such as the High Altitude Observatory's Mark IV Coronagraph on top of Mauna Loa, use polarization to distinguish sky brightness from the image of the corona: both coronal light and sky brightness are scattered sunlight and have similar spectral properties, but the coronal light is Thomson-scattered at nearly a right angle and therefore undergoes scattering polarization, while the superimposed light from the sky is scattered at only a glancing angle and hence remains nearly unpolarized.

Space-based coronagraphs such as LASCO avoid the sky brightness problem; the next challenge becomes stray light management under the stringent size and weight requirements of space flight. Any sharp edge (such as the edge of an occulting disk or optical aperture) causes Fresnel diffraction of incoming light around the edge, which imposes a strict relationship between the size of an instrument and the amount of stray light that leaks around its aperture. The LASCO C-3 coronagraph uses both an external occulter (which casts shadow on the instrument) and an internal occulter (which blocks stray light that is Fresnel-diffracted around the external occulter) to reduce this "leakage", and a complicated system of baffles to eliminate stray light scattering off the internal surfaces of the instrument itself.

1 Other example images

[edit] Other example images

Here are some example images from NASA's web site.

A comet caught by a sun-facing observatory - NASA

A simulated view of the coronagraph for Terrestrial Planet Finder- Courtesy NASA/JPL-Caltech

[edit] Nulling Coronagraph

A nulling coronagraph uses a mask to shift the phase of light, rather than a simple opaque disc to block it. [2]

[edit] Optical Vortex Coronagraph

Described in the Astrophysical Journal issue of November 10, 2005 by Dimitri Mawet, Pierre Riaud, Olivier Absil and Jean Surdej from the University of Liège, an Annular Groove Phase Mask Coronagraph. It is made up of a concentric circular subwavelength grating that induces an achromatic optical vortex. This coronagraphic device could be used alone on a classical telescope but also with an interferometer. See [3] See [4]

Described in December 2005 by Gregory Foo, David M. Palacios, Grover A. Swartzlander Jr. at the University of Arizona, Tucson, an Optical Vortex Coronagraph, uses a helical phase mask to null the light arriving on-axis. See [5], [6]