Acoronagraph is atelescopic attachment designed to block out the direct light from astar or other bright object so that nearby objects – which otherwise would be hidden in the object's brightglare – can be resolved. Most coronagraphs are intended to view thecorona of theSun, but a new class of conceptually similar instruments (calledstellar coronagraphs to distinguish them fromsolar coronagraphs) are being used to findextrasolar planets andcircumstellar disks around nearby stars as well as host galaxies inquasars and other similar objects withactive galactic nuclei (AGN).
The coronagraph was introduced in 1931 by the French astronomerBernard Lyot; since then, coronagraphs have been used at manysolar observatories. Coronagraphs operating withinEarth's atmosphere suffer from scattered light in thesky itself, due primarily toRayleigh 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 theHigh Altitude Observatory'sMark IV Coronagraph on top ofMauna Loa, usepolarization to distinguish sky brightness from the image of the corona: both coronal light andsky brightness are scatteredsunlight and have similar spectral properties, but the coronal light isThomson-scattered at nearly aright angle and therefore undergoesscattering polarization, while the superimposed light from the sky near the Sun is scattered at only a glancing angle and hence remains nearly unpolarized.
Coronagraph instruments are extreme examples ofstray light rejection and precisephotometry because the total brightness from the solar corona is less than one-millionth the brightness of the Sun.[1] 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 atotal solar eclipse, theMoon acts as an occluding 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 intermediatefocal 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 anddiffraction 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 asLyot stops, 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.[2]
As examples, imaging instruments on theHubble Space Telescope andJames Webb Space Telescope offer coronagraphic capability.
Aband-limited coronagraph uses a special kind of mask called aband-limited mask.[3] 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 canceledTerrestrial Planet Finder coronagraph. Band-limited masks are available on theJames Webb Space Telescope as well.
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.
Anoptical vortex 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 they are so far away their light is, for this purpose, a spatially 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.
Coronagraphs inouter 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 asNASA-ESA'sSOHO, and NASA's SPARTAN,Solar Maximum Mission, andSkylab have used coronagraphs to study the outer reaches of the solar corona. TheHubble Space Telescope (HST) is able to perform coronagraphy using theNear Infrared Camera and Multi-Object Spectrometer (NICMOS),[6] and theJames Webb Space Telescope (JWST) is able to perform coronagraphy using theNear Infrared Camera (NIRCam) andMid-Infrared Instrument (MIRI).
While space-based coronagraphs such asLASCO 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) causesFresnel diffraction of incoming light around the edge, which means that the smaller instruments that one would want on a satellite unavoidably 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.
Aditya-L1 is a coronagraphy spacecraft developed by theIndian Space Research Organisation (ISRO) and various Indian research institutes. The spacecraft aims to study the solar atmosphere and its impact on the Earth's environment. It will be positioned approximately 1.5 million km from Earth in a halo orbit around the L1Lagrangian point between Earth and the Sun.[7][8]
The primary payload, Visible Emission Line Coronagraph (VELC), will send 1,440 images of the sun daily to ground stations. The VELC payload has been developed by theIndian Institute of Astrophysics (IIA) and will continuously observe the Sun's corona from the L1 point.[8][9]
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 1900arcseconds, while a typical nearby star might have an apparent size of 0.0005 and 0.002 arcseconds.) Earth-like exoplanet detection requires 10−10 contrast.[10] To achieve such contrast requires extremeoptothermal stability.
A stellar coronagraph concept was studied for flight on the canceledTerrestrial Planet Finder mission. On ground-based telescopes, a stellar coronagraph can be combined withadaptive optics to search for planets around nearby stars.[11]
In November 2008, NASA announced that a planet was directly observed orbiting the nearby starFomalhaut. The planet could be seen clearly on images taken by Hubble's Advanced Camera for Surveys' coronagraph in 2004 and 2006.[12] The dark area hidden by the coronagraph mask can be seen on the images, and a bright dot has been added to represent the location of the star itself.
Up until the year 2010,telescopes could onlydirectly image exoplanets under exceptional circumstances. Specifically, it is easier to obtain images when the planet is especially large (considerably larger thanJupiter), widely separated from its parent star, and hot so that it emits intense infrared radiation. However, in 2010 a team fromNASA'sJet Propulsion Laboratory demonstrated that a vector vortex coronagraph could enable small telescopes to directly image planets.[13] They did this by imaging the previously imagedHR 8799 planets using just a1.5 m portion of theHale Telescope.
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