coronagraph

A coronagraph is a telescopic attachment designed to block out the direct light from a star so that nearby objects – which otherwise would be hidden in the star's bright glare – can be resolved. 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.

A comet caught by a sun-facing observatory - NASA

Invention

The coronagraph was introduced in 1930 by the french 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 near the Sun is scattered at only a glancing angle and hence remains nearly unpolarized.

Design

Coronagraph instruments are extreme examples of 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. More common is an arrangement where the sky is imaged onto an intermediate focal plane containing an opaque spot; this focal plane is reimaged onto a detector. Another arrangement is to image the sky onto a mirror with a small hole: the desired light is reflected and eventually reimaged, but the unwanted light from the star goes through the hole and does not reach the detector. Either way, the instrument design must take into account scattering and diffraction to make sure that as little unwanted light as possible reaches the final detector. As an example, imaging instruments on the Hubble Space Telescope offer coronagraphic capability.

Band-limited coronagraph

A band-limited coronagraph uses a special kind of mask called a band-limited mask.^[1] 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: [1], [2], [3]

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

Some Vortex Phase Mask exist:

• the Optical Vortex Coronagraph based on a phase ramp on a dielectric material, like fused silica. See also: [4], [5].

• the Annular Groove Phase Mask based on a mask made up of a concentric circular subwavelength grating that induces an achromatic optical vortex. See also: [6]

Satellite-based coronographs

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),^[2] and there are plans to have this capability on the James Webb Space Telescope (JWST) using its Near Infrared Camera (NIRCam) and Mid Infrared Instrument (MIRI).

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, as and when that mission is eventually launched. On ground-based telescopes, a stellar coronagraph can be combined with adaptive optics to search for planets around nearby stars [7].

Direct observation of extrasolar planets

In 2008 November, NASA announced that a planet was directly observed orbiting the nearby star Fomalhaut. The planet could be seen clearly on images taken by Hubble's Advanced Camera for Surveys' coronagraph in 2004 and 2006 [8]. The dark area hidden by the coronagraph mask can be seen on the images, though a bright dot has been added "by hand" to show where the star would have been.

See also: Starshade - A proposed Coronograph

Example images

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

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

This link shows an HST image of a dust disk surrounding a bright star with the star hidden by the coronagraph.

References

1. ^ "A Coronagraph with a Band-limited Mask for Finding Terrestrial Planets". Astrophysical Journal. 2002-05-10. http://www.journals.uchicago.edu/doi/full/10.1086/339625. 
2. ^ NICMOS Coronagraphy

See also

• List of solar telescopes