Ajay Kumar at 05:43, 2 September 2023

2023-09-02T05:43:21Z

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{{Short description|Telescopic attachment designed to block out the direct light from a star}}
[[File:Cp22liberationdaytransient.png|thumb|Coronagraph image of the Sun]]
A '''coronagraph''' is a [[Telescope|telescopic]] attachment designed to block out the direct light from a [[star]] or other bright object so that nearby objects – which otherwise would be hidden in the object's bright [[Glare (vision)|glare]] – can be resolved. Most coronagraphs are intended to view the [[solar corona|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 and [[circumstellar disk]]s around nearby stars as well as host galaxies in [[quasar]]s and other similar objects with [[active galactic nuclei]] (AGN).

==Invention==
The coronagraph was introduced in 1931 by the French astronomer [[Bernard Lyot]]; since then, coronagraphs have been used at many [[Solar observatory|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 (waves)|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 [[Polarization (waves)#Propagation, reflection and scattering|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==
[[File: Wendelstein Solar Telescope.jpg|thumb|Coronagraph at the [[Wendelstein (mountain)#Mountain weather station and observatory|Wendelstein Observatory]]]]
Coronagraph instruments are extreme examples of [[stray light]] rejection and precise [[photometry (astronomy)|photometry]] because the total brightness from the solar corona is less than one-millionth 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|total solar eclipse]], the [[Moon]] 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 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.<ref>{{Cite web|url=https://umbra.nascom.nasa.gov/spartan/coronagraphs.html|title=SPARTAN 201-3: Coronagraphs|website=umbra.nascom.nasa.gov|access-date=2020-03-30}}</ref>

As examples, imaging instruments on the [[Hubble Space Telescope]] and [[James Webb Space Telescope]] offer coronagraphic capability.

===Band-limited coronagraph===
A ''band-limited coronagraph'' uses a special kind of mask called a ''band-limited mask''.<ref name="band-limited">{{cite journal | url=http://www.iop.org/EJ/article/0004-637X/570/2/900/54598.html | author=Kuchner and Traub | title=A Coronagraph with a Band-limited Mask for Finding Terrestrial Planets | journal=The Astrophysical Journal | volume=570 | issue=2 | year=2002 | pages=900–908 | doi=10.1086/339625 | bibcode=2002ApJ...570..900K|arxiv = astro-ph/0203455 | s2cid=18095697 }}</ref> 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 canceled [[Terrestrial Planet Finder]] coronagraph. Band-limited masks will also be available on the [[James Webb Space Telescope]].

===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.

===Optical vortex coronagraph===
{{Main|Vortex coronagraph}}
An [[optical vortex]] 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.<ref>{{Cite journal|last1=Foo|first1=Gregory|last2=Palacios|first2=David M.|last3=Swartzlander|first3=Grover A. Jr.|date=December 15, 2005|title=Optical vortex coronagraph|url=http://www.cis.rit.edu/~grovers/Doc/Publications/OVC.pdf|journal=Optics Letters|volume=30|issue=24|pages=3308–3310|doi=10.1364/OL.30.003308|pmid=16389814|bibcode=2005OptL...30.3308F }}</ref><ref>[http://www.u.arizona.edu/~grovers/ovc.html Optical vortex coronagraph] {{webarchive|url=https://web.archive.org/web/20060903215113/http://www.u.arizona.edu/~grovers/ovc.html |date=2006-09-03 }}</ref>
* 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 polymer]] (same technology as in [[3D television]]), and micro-structured surfaces (using [[microfabrication]] technologies from the [[microelectronics]] industry). Such a vector vortex coronagraph made out of liquid crystal polymers is currently in use at the 200-inch [[Hale telescope]] at the [[Palomar Observatory]]. It has recently been operated with [[adaptive optics]] to image [[extrasolar planets]].

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.

==Satellite-based coronagraphs==
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 [[Solar and Heliospheric Observatory|SOHO]], and NASA's SPARTAN, [[Solar Maximum Mission]], 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),<ref>{{Cite web|url=http://www.stsci.edu/home/hst/instrumentation/legacy/nicmos|title=NICMOS|website=STScI.edu|language=en|access-date=2020-03-30}}</ref> and the [[James Webb Space Telescope]] (JWST) is able to perform coronagraphy using the [[Near Infrared Camera]] (NIRCam) and [[Mid-Infrared Instrument]] (MIRI).

While space-based coronagraphs such as [[Solar and Heliospheric Observatory#Instruments|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 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.

==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 [[arcsecond]]s, 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.<ref>{{Cite journal|last1=Brooks|first1=Thomas|last2=Stahl|first2=H. P.|last3=Arnold|first3=William R.|editor1-first=Mark A|editor1-last=Kahan|editor2-first=Marie B|editor2-last=Levine-West|date=2015-09-23|title=Advanced Mirror Technology Development (AMTD) thermal trade studies|url=http://spie.org/Publications/Proceedings/Paper/10.1117/12.2188371?origin_id=x4323&start_year=1963|journal=Optical Modeling and Performance Predictions VII|volume=9577|page=957703|publisher=SPIE|doi=10.1117/12.2188371|bibcode=2015SPIE.9577E..03B |hdl=2060/20150019495|s2cid=119544105 |hdl-access=free}}</ref> To achieve such contrast requires extreme [[optothermal stability]].

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.<ref>{{Cite web|url=http://www.adaptiveoptics.org/News_0805_1.html|title=Gemini Observatory Board Goes Forward with Extreme Adaptive Optics Coronagraph|website=www.adaptiveoptics.org|access-date=2020-03-30}}</ref>


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.<ref>{{Cite web|url=https://www.nasa.gov/mission_pages/hubble/science/fomalhaut.html|title=NASA - Hubble Directly Observes a Planet Orbiting Another Star|website=www.nasa.gov|language=en|access-date=2020-03-30}}</ref> 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.

[[File:444226main exoplanet20100414-a-full.jpg|250px|thumb|Direct image of [[exoplanet]]s around the star [[HR8799]] using a [[vector vortex coronagraph]] on a 1.5 m portion of the [[Hale telescope]]]]
Up until the year 2010, [[telescope#Types|telescopes]] could only [[Methods of detecting extrasolar planets#Direct imaging|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.<ref>{{Cite web|url=http://www.nbcnews.com/id/36528711/ns/technology_and_science-space/t/new-method-could-image-earth-like-planets/|title=New method could image Earth-like planets|last=Andrea Thompson|date=2010-04-14|website=msnbc.com|language=en|access-date=2020-03-30}}</ref> They did this by imaging the previously imaged [[HR 8799]] planets using just a {{Val|1.5|u=m}} portion of the [[Hale Telescope]].

==See also==
*[[List of solar telescopes]]
*[[New Worlds Mission]] – A proposed external coronagraph

==References==
{{Reflist}}

==External links==
* [https://web.archive.org/web/20111015153123/http://exep.jpl.nasa.gov/files/exep/TechnologyAstro2010WP-VisiblePlanetImagingFinal.pdf Overview of Technologies for Direct Optical Imaging of Exoplanets], Marie Levine, Rémi Soummer, 2009
* [https://books.google.com/books?id=8dwDAAAAMBAJ&pg=PA140 "Sun Gazer's Telescope."] ''Popular Mechanics'', February 1952, pp. 140–141. Cut-away drawing of first Coronagraph type used in 1952.
* [http://www.opticsinfobase.org/abstract.cfm?URI=oe-17-3-1902 Optical Vectorial Vortex Coronagraphs using Liquid Crystal Polymers: theory, manufacturing and laboratory demonstration] Optics Infobase
* [http://iopscience.iop.org/0004-637X/709/1/53/ The Vector Vortex Coronagraph: Laboratory Results and First Light at Palomar Observatory] IopScience
* [http://iopscience.iop.org/0004-637X/633/2/1191/ Annular Groove Phase Mask Coronagraph] IopScience
*This link shows an HST image of a dust disk surrounding a bright star with the star hidden by the coronagraph.[http://hubblesite.org/newscenter/archive/releases/2003/02/image/a]

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[[Category:Optical telescope components]]
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Coronagraph - Revision history

Ajay Kumar at 05:43, 2 September 2023