Coronagraph
Encyclopedia
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 planet
s around nearby stars.
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.
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.
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. Lyot's key invention was an arrangement of lenses with stops, known as Lyot stop
s, and baffles such that light scattered by diffraction was focused on the stops and baffles, where it could be absorbed, while light needed for a useful image missed them.
As an example, imaging instruments on the Hubble Space Telescope
offer coronagraphic capability.
coronagraph. Band-limited masks will also be available on the James Webb Space Telescope
.
See also: http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v594n1/56512/brief/56512.abstract.html, http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v628n1/61581/brief/61581.abstract.html, http://www.nature.com/nature/journal/v446/n7137/abs/nature05729.html
coronagraph uses a phase-mask in which the phase-shift varies azimuthally around the center. Several varieties of optical vortex coronagraphs exist:
This works with stars, other than the sun because, since they are so far away their light is, for this purpose, a spacially coherent plane wave. The coronagraph, using interference, masks out the light along the center axis of the telescope, but allows the light from off axis objects through.
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), 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 means that the smaller instruments that one would want on a satellite unavoidable leak more light than larger ones would. 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.
A stellar coronagraph concept was studied for flight on the canceled Terrestrial Planet Finder
mission. On ground-based telescopes, a stellar coronagraph can be combined with adaptive optics
to search for planets around nearby stars http://www.adaptiveoptics.org/News_0805_1.html.
This link shows an HST image of a dust disk surrounding a bright star with the star hidden by the coronagraph.
In November 2008, 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 http://www.nasa.gov/mission_pages/hubble/science/fomalhaut.html. The dark area hidden by the coronagraph mask can be seen on the images, though a bright dot has been added to show where the star would have been.
Up until the year 2010, telescopes could only directly image exoplanets under exceptional circumstances. Specifically, it is easier to obtain images when the planet is especially large (considerably larger than Jupiter
), widely separated from its parent star, and hot so that it emits intense infrared radiation. However in 2010 a team from NASA
s Jet Propulsion Laboratory
demonstrated that a vector vortex coronagraph could enable small telescopes to directly image planets. They did this by imaging the previously imaged HR 8799
planets using just a 1.5 m portion of the Hale Telescope
.
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
attachment designed to block out the direct light from a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
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
Corona
A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...
of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, but a new class of conceptually similar instruments (called stellar coronagraphs to distinguish them from solar coronagraphs) are being used to find extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
s around nearby stars.
Invention
The coronagraph was introduced in 1930 by the French astronomerAstronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...
Bernard Lyot
Bernard Lyot
Bernard Ferdinand Lyot was a French astronomer.His interest in astronomy started in 1914. He soon acquired a telescope and soon upgraded to a . From graduation in 1918 until 1929, he worked as a demonstrator at the Ecole Polytechnique...
; since then, coronagraphs have been used at many solar observatories
Observatory
An observatory is a location used for observing terrestrial or celestial events. Astronomy, climatology/meteorology, geology, oceanography and volcanology are examples of disciplines for which observatories have been constructed...
. Coronagraphs operating within Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
's atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
suffer from scattered light in the sky
Sky
The sky is the part of the atmosphere or outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight, the sky of Earth has the appearance of a pale blue surface because the air scatters the sunlight. The sky is sometimes...
itself, due primarily to Rayleigh scattering
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
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
High Altitude Observatory
The High Altitude Observatory conducts research and provides support and facilities for the solar-terrestrial research community in the areas of solar and heliospheric physics, and the effects of solar variability on the Earth's magnetosphere, ionosphere, and upper atmosphere.HAO is a laboratory...
's Mark IV Coronagraph on top of Mauna Loa
Mauna Loa
Mauna Loa is one of five volcanoes that form the Island of Hawaii in the U.S. state of Hawaii in the Pacific Ocean, and the largest on Earth in terms of volume and area covered. It is an active shield volcano, with a volume estimated at approximately , although its peak is about lower than that...
, use polarization to distinguish sky brightness from the image of the corona: both coronal light and sky brightness are scattered sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
and have similar spectral properties, but the coronal light is Thomson-scattered at nearly a right angle
Right angle
In geometry and trigonometry, a right angle is an angle that bisects the angle formed by two halves of a straight line. More precisely, if a ray is placed so that its endpoint is on a line and the adjacent angles are equal, then they are right angles...
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 lightStray light
Stray light is light in an optical system, which was not intended in the design. The light may be from the intended source, but follow paths other than intended, or it may be from a source other than the intended source...
rejection and precise photometry
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
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.
During a solar eclipse
Solar eclipse
As seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and the Earth, and the Moon fully or partially blocks the Sun as viewed from a location on Earth. This can happen only during a new moon, when the Sun and the Moon are in conjunction as seen from Earth. At least...
, the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
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
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
to make sure that as little unwanted light as possible reaches the final detector. Lyot's key invention was an arrangement of lenses with stops, known as Lyot stop
Lyot stop
A Lyot stop is an optic stop, invented by French astronomer Bernard Lyot, that reduces the amount of flare caused by diffraction of other stops and baffles in optical systems.-External links:** by Bass et al....
s, and baffles such that light scattered by diffraction was focused on the stops and baffles, where it could be absorbed, while light needed for a useful image missed them.
As an example, imaging instruments on the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
offer coronagraphic capability.
Band-limited coronagraph
A band-limited coronagraph uses a special kind of mask called a band-limited mask. 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 FinderTerrestrial Planet Finder
The Terrestrial Planet Finder was a proposed project by NASA to construct a system of telescopes for detecting extrasolar terrestrial planets. TPF was postponed several times and finally cancelled...
coronagraph. Band-limited masks will also be available on the James Webb Space Telescope
James Webb Space Telescope
The James Webb Space Telescope , previously known as Next Generation Space Telescope , is a planned next-generation space telescope, optimized for observations in the infrared. The main technical features are a large and very cold 6.5 meter diameter mirror, an observing position far from Earth,...
.
See also: http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v594n1/56512/brief/56512.abstract.html, http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v628n1/61581/brief/61581.abstract.html, http://www.nature.com/nature/journal/v446/n7137/abs/nature05729.html
Phase-mask coronagraph
A phase-mask coronagraph (such as the so-called four-quadrant phase-mask coronagraph) uses a transparent mask to shift the phase of the stellar light in order to create a self-destructive interference, rather than a simple opaque disc to block it. See also: http://www.journals.uchicago.edu/doi/abs/10.1086/317707. Phase-apodization coronagraphy, uses an unconventional mathematical approach to model the phase plate. To block out glare from a star, conventional coronagraphs have to be precisely lined up and are highly susceptible to disturbance. An apodising phase-plate coronagraph, on the other hand, requires no aiming and works equally well on any stars or locations in the image.http://www.optoiq.com/index/photonics-technologies-applications/lfw-display/lfw-article-display/6781945422/articles/optoiq2/photonics-technologies/news/applications-__markets/science-__research/astronomy-__astrophysics/2010/10/exoplanet-hunters.htmlOptical vortex coronagraph
An optical vortexOptical vortex
An optical vortex is a zero of an optical field, a point of zero intensity. Research into the properties of vortices has thrived since a comprehensive paper by Nye and Berry, in 1974, described the basic properties of "dislocations in wave trains"...
coronagraph uses a phase-mask in which the phase-shift varies azimuthally around the center. Several varieties of optical vortex coronagraphs exist:
- the scalar optical vortex coronagraph based on a phase ramp directly etched in a dielectric material, like fused silica.
- the vector(ial) vortex coronagraph employs a mask that rotates the angle of polarization of photons, and ramping this angle of rotation has the same effect as ramping a phase-shift. A mask of this kind can be synthesized by various technologies, ranging from liquid crystal polymerLiquid crystal polymerLiquid-crystal polymers are a class of aromatic polyester polymers. They are extremely unreactive and inert, and highly resistant to fire.-Background:...
(same technology as in 3D television3D televisionA 3D television is a television set that employs techniques of 3D presentation, such as stereoscopic capture, multi-view capture, or 2D-plus-depth, and a 3D display – a special viewing device to project a television program into a realistic three-dimensional field.- History :In the late-1890's,...
), and micro-structured surfaces (using microfabricationMicrofabricationMicrofabrication is the term that describes processes of fabrication of miniature structures, of micrometre sizes and smaller. Historically the earliest microfabrication processes were used for integrated circuit fabrication, also known as "semiconductor manufacturing" or "semiconductor device...
technologies from the microelectronicsMicroelectronicsMicroelectronics is a subfield of electronics. As the name suggests, microelectronics relates to the study and manufacture of very small electronic components. Usually, but not always, this means micrometre-scale or smaller,. These devices are made from semiconductors...
industry). Such a vector vortex coronagraph made out of liquid crystal polymers is currently in use at the 200-inch Hale telescopeHale telescopeThe Hale Telescope is a , 3.3 reflecting telescope at the Palomar Observatory in California, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation, he orchestrated the planning, design, and construction of the observatory, but did not live to see its commissioning...
at the Palomar ObservatoryPalomar ObservatoryPalomar Observatory is a privately owned observatory located in San Diego County, California, southeast of Pasadena's Mount Wilson Observatory, in the Palomar Mountain Range. At approximately elevation, it is owned and operated by the California Institute of Technology...
. It has recently been operated with adaptive opticsAdaptive opticsAdaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...
to image extrasolar planets.
This works with stars, other than the sun because, since they are so far away their light is, for this purpose, a spacially coherent plane wave. The coronagraph, using interference, masks out the light along the center axis of the telescope, but allows the light from off axis objects through.
Satellite-based coronagraphs
Coronagraphs in outer spaceOuter space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....
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
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
-ESA's SOHO
Solar and Heliospheric Observatory
The Solar and Heliospheric Observatory is a spacecraft built by a European industrial consortium led by Matra Marconi Space that was launched on a Lockheed Martin Atlas IIAS launch vehicle on December 2, 1995 to study the Sun, and has discovered over 2100 comets. It began normal operations in May...
, SPARTAN, and Skylab
Skylab
Skylab was a space station launched and operated by NASA, the space agency of the United States. Skylab orbited the Earth from 1973 to 1979, and included a workshop, a solar observatory, and other systems. It was launched unmanned by a modified Saturn V rocket, with a mass of...
have used coronagraphs to study the outer reaches of the solar corona. The Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
(HST) is able to perform coronagraphy using the Near Infrared Camera and Multi-Object Spectrometer
Near Infrared Camera and Multi-Object Spectrometer
The Near Infrared Camera and Multi-Object Spectrometer is a scientific instrument for infrared astronomy, installed on the Hubble Space Telescope , operating from 1997 to 1999, and from 2002 to 2008...
(NICMOS), and there are plans to have this capability on the James Webb Space Telescope
James Webb Space Telescope
The James Webb Space Telescope , previously known as Next Generation Space Telescope , is a planned next-generation space telescope, optimized for observations in the infrared. The main technical features are a large and very cold 6.5 meter diameter mirror, an observing position far from Earth,...
(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
Fresnel diffraction
In optics, the Fresnel diffraction equation for near-field diffraction, is an approximation of Kirchhoff-Fresnel diffraction that can be applied to the propagation of waves in the near field....
of incoming light around the edge, which means that the smaller instruments that one would want on a satellite unavoidable leak more light than larger ones would. 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.
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 1900 arcseconds, while a typical nearby star might have an apparent size of 0.0005 and 0.002 arcseconds.)A stellar coronagraph concept was studied for flight on the canceled Terrestrial Planet Finder
Terrestrial Planet Finder
The Terrestrial Planet Finder was a proposed project by NASA to construct a system of telescopes for detecting extrasolar terrestrial planets. TPF was postponed several times and finally cancelled...
mission. On ground-based telescopes, a stellar coronagraph can be combined with adaptive optics
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...
to search for planets around nearby stars http://www.adaptiveoptics.org/News_0805_1.html.
This link shows an HST image of a dust disk surrounding a bright star with the star hidden by the coronagraph.
In November 2008, 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 http://www.nasa.gov/mission_pages/hubble/science/fomalhaut.html. The dark area hidden by the coronagraph mask can be seen on the images, though a bright dot has been added to show where the star would have been.
Up until the year 2010, telescopes could only directly image exoplanets under exceptional circumstances. Specifically, it is easier to obtain images when the planet is especially large (considerably larger than Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
), widely separated from its parent star, and hot so that it emits intense infrared radiation. However in 2010 a team from NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
s Jet Propulsion Laboratory
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...
demonstrated that a vector vortex coronagraph could enable small telescopes to directly image planets. They did this by imaging the previously imaged HR 8799
HR 8799
HR 8799 is a young main sequence star located 129 light years away from Earth in the constellation of Pegasus, with roughly 1.5 times the Sun's mass and 4.9 times its luminosity. It is part of a system that also contains a debris disk and at least four massive planets...
planets using just a 1.5 m portion of the Hale Telescope
Hale telescope
The Hale Telescope is a , 3.3 reflecting telescope at the Palomar Observatory in California, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation, he orchestrated the planning, design, and construction of the observatory, but did not live to see its commissioning...
.
External links
- http://www.opticsinfobase.org/abstract.cfm?URI=oe-17-3-1902 Optics Infobase, *http://iopscience.iop.org/0004-637X/709/1/53/ IopScience
- http://iopscience.iop.org/0004-637X/633/2/1191/ IopScience