coronagraph

 

(kərō'nəgrăf') , device invented by the French astronomer B. Lyot (1931) for the purpose of observing the corona of the sun and solar prominences occurring in the chromosphere. Because of the intense light of the sun, the corona and chromosphere can ordinarily be seen only during a total eclipse. The coronagraph consists of two refracting telescopes in tandem. A solid disk placed in front of the prime focus of the first telescope plays the part of the moon and eclipses the sun's image in the telescope so that only the outer layers of the sun's atmosphere are focused by the second telescope onto photographic film. A monochromatic filter is also used to improve optical clarity and remove chromatic aberration.

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, but a new class of conceptually similar instruments (called stellar coronagraphs to distinguish them from solar coronagraphs) are being used to find extrasolar planets around nearby stars.

The coronagraph was introduced in 1930 by the astronomer Bernard Lyot; since then, coronagraphs have been used at many solar observatories. 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.

Coronagraph instruments are studies in stray light rejection and precise photometry, 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.

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 in outer space are much more effective than the same instruments would be, if located on the ground. This is because the complete absence of atmospheric scattering eliminates the largest source of glare present in a terrestrial coronagraph. Several space missions such as NASA-ESA's SOHO, SPARTAN, and Skylab have used coronagraphs to study the outer reaches of the solar corona. The Hubble Space Telescope (HST) is able to perform coronagraphy using the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) ^[1], and there are plans to have this capability on the James Webb Space Telescope (JWST) using its Near Infrared Camera (NIRCam).

While space-based coronagraphs such as LASCO avoid the sky brightness problem, they face design challenges in 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.

Coronagraphs for Studying Other Stars and Extrasolar Planets

The coronagraph has recently been adapted to the challenging task of finding planets around nearby stars. While stellar and solar coronagraphs are similar in concept, they are quite different in practice because the object to be occulted differs by a factor of a million in linear apparent size (the Sun has an apparent size of about 1,800 arcseconds, while a typical nearby star might have an apparent size of 0.5-2 milliarcseconds).

A stellar coronagraph concept is currently being studied to fly on the Terrestrial Planet Finder mission. On ground-based telescopes, a stellar coronagraph can be combined with adaptive optics to search for planets around nearby stars [1].

Band-Limited Coronagraph

A band-limited coronagraph uses a special kind of mask called a band-limited mask. ^[2] This mask is designed to block light and also manage diffraction effects caused by removal of the light. The band-limited coronagraph has served as the baseline design for the Terrestrial Planet Finder coronagraph.

See also: [2], [3], [4]

Nulling Coronagraph

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

Optical Vortex Coronagraph

An Optical Vortex Coronagraph is based on a mask made up of a concentric circular subwavelength grating that induces an achromatic optical vortex. ^[3]

See also: [5], [6], [7], [8].

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

See also

• Starshade - A proposed Coronograph

References

1. ^ NICMOS Coronagraphy
2. ^ "A Coronagraph with a Band-limited Mask for Finding Terrestrial Planets", Astrophysical Journal, May 10, 2002. 
3. ^ "Annular Groove Phase Mask Coronagraph", Astrophysical Journal, November 10, 2005. 

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