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 and circumstellar disks around nearby stars.

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Coronagraph image of the Sun

Invention

The coronagraph was introduced in 1931 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

Wendelstein Solar Telescope
Coronagraph at the Wendelstein Observatory

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.

During a 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 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.[1]

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.[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 canceled Terrestrial Planet Finder coronagraph. Band-limited masks will also be available on the James Webb Space Telescope.

See also: [2]

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: [3] [4]

Optical 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.[3][4]
  • 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 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),[5] 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 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 arcseconds, 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 [5]. 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 [6].

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 [7]. 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.

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Direct image of exoplanets around the star HR8799 using a vector vortex coronagraph on a 1.5m portion of the Hale telescope

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 NASAs Jet Propulsion Laboratory demonstrated that a vector vortex coronagraph could enable small telescopes to directly image planets.[6] They did this by imaging the previously imaged HR 8799 planets using just a 1.5 m portion of the Hale Telescope.

See also

References

  1. ^ [1] NASA page with diagrams of coronagraphs
  2. ^ Kuchner and Traub (2002). "A Coronagraph with a Band-limited Mask for Finding Terrestrial Planets". The Astrophysical Journal. 570 (2): 900–908. arXiv:astro-ph/0203455. Bibcode:2002ApJ...570..900K. doi:10.1086/339625.
  3. ^ optical vortex coronagraph
  4. ^ Optical vortex coronagraph Archived 2006-09-03 at the Wayback Machine
  5. ^ NICMOS Coronagraphy
  6. ^ New method could image Earth-like planets

External links

Astrophysics Strategic Mission Concept Studies

Astrophysics Strategic Mission Concept Studies [AMSCS] is an program within the National Aeronautics and Space Administration agency of the United States government for possible projects leading to probable prospective missions.

Coronal mass ejection

A coronal mass ejection (CME) is a significant release of plasma and accompanying magnetic field from the solar corona. They often follow solar flares and are normally present during a solar prominence eruption. The plasma is released into the solar wind, and can be observed in coronagraph imagery.Coronal mass ejections are often associated with other forms of solar activity, but a broadly accepted theoretical understanding of these relationships has not been established. CMEs most often originate from active regions on the Sun's surface, such as groupings of sunspots associated with frequent flares. Near solar maxima, the Sun produces about three CMEs every day, whereas near solar minima, there is about one CME every five days.

Exoplanetary Circumstellar Environments and Disk Explorer

Exoplanetary Circumstellar Environments and Disk Explorer (EXCEDE) is a proposed space telescope for NASA's Explorer program to observe circumstellar protoplanetary and debris discs and study planet formation around nearby (within 100 parsecs) stars of spectral classes M to B. Had it been selected for development, it was proposed to launch in 2019.

The spacecraft concept proposed to use a 70 centimeter diameter telescope-mounted coronagraph called PIAA (Phase Induced Amplitude Apodized Coronagraph) to suppress starlight in order to be able to detect fainter radiation of circumstellar dust. Characterizing constitution of such disks would provide clues for planetary formation (mostly in habitable zones), while already existing exoplanets can be detected through their interaction with dust disk. The project's Principal Investigator is Glenn Schneider.

Gemini Planet Imager

The Gemini Planet Imager (GPI) is a high contrast imaging instrument that was built for the Gemini South Telescope in Chile. The instrument achieves high contrast at small angular separations, allowing for the direct imaging and integral field spectroscopy of extrasolar planets around nearby stars. The collaboration involved in planning and building the Gemini Planet imager includes the American Museum of Natural History (AMNH), Dunlap Institute, Gemini Observatory, Herzberg Institute of Astrophysics (HIA), Jet Propulsion Laboratory, Lawrence Livermore National Lab (LLNL), Lowell Observatory, SETI Institute, The Space Telescope Science Institute (STSCI), the University of Montreal, University of California, Berkeley, University of California, Los Angeles (UCLA), University of California, Santa Cruz (UCSC), University of Georgia.

HR 8799

HR 8799 is a roughly 30 million-year-old main-sequence star located 129 light-years (39.6 parsecs) away from Earth in the constellation of Pegasus. It has 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. Those planets, along with Fomalhaut b, were the first extrasolar planets whose orbital motion was confirmed via direct imaging. The designation HR 8799 is the star's identifier in the Bright Star Catalogue. The star is a Gamma Doradus variable: its luminosity changes because of non-radial pulsations of its surface. The star is also classified as a Lambda Boötis star, which means its surface layers are depleted in iron peak elements. This may be due to the accretion of metal-poor circumstellar gas. It is the only known star which is simultaneously a Gamma Doradus variable, a Lambda Boötis type, and a Vega-like star (a star with excess infrared emission caused by a circumstellar disk).

Habitable Exoplanet Imaging Mission

The Habitable Exoplanet Imaging Mission (HabEx) is a space telescope concept that would be optimized to search for and image Earth-size habitable exoplanets in the habitable zones of their stars, where liquid water can exist. HabEx would aim to understand how common terrestrial worlds beyond the Solar System may be and the range of their characteristics. It would be an optical, UV and infrared telescope that would also use spectrographs to study planetary atmospheres and eclipse starlight with either an internal coronagraph or an external starshade.The preliminary concept, which is still in formulation, was proposed in 2016 to become the next Flagship (Large Strategic Science Mission) space observatory. If selected in 2020 and launched, it will operate at Lagrangian point L2.

Large Angle and Spectrometric Coronagraph

The Large Angle and Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory satellite (SOHO) consists of three solar coronagraphs with nested fields of view:

C1 - a Fabry–Pérot interferometer coronagraph imaging from 1.1 to 3 solar radii

C2 - a white light coronagraph imaging from 1.5 to 6 solar radii (orange)

C3 - a white light coronagraph imaging from 3.7 to 30 solar radii (blue)The first principal investigator was Dr. Guenter Brueckner. These coronagraphs monitor the solar corona by using an optical system to create, in effect, an artificial solar eclipse. The white light coronagraphs C2 and C3 produce images of the corona over much of the visible spectrum, while the C1 interferometer produces images of the corona in a number of very narrow visible wavelength bands.

Large UV Optical Infrared Surveyor

The Large UV Optical Infrared Surveyor (LUVOIR) is a multi-wavelength space observatory concept being developed by NASA under the leadership of a Science and Technology Definition Team drawn from the scientific and technical community.

LUVOIR is one of four large astrophysics space mission concepts being studied in preparation for the National Academy of Sciences 2020 Astronomy and Astrophysics Decadal Survey.While LUVOIR is a concept for a general-purpose observatory, it has the key science goal of characterizing a wide range of exoplanets, including those that might be habitable.

An additional goal is to enable a broad range of astrophysics, from the reionization epoch, through galaxy formation and evolution, to star and planet formation.

Powerful imaging and spectroscopy observations of Solar System bodies would also be possible.

LUVOIR would be a Large Strategic Science Mission and will be considered for a development start sometime after 2020.

The LUVOIR Team has produced designs for two variants of LUVOIR: one with a 15 m diameter telescope mirror (LUVOIR-A) and one with an 8 m diameter mirror (LUVOIR-B).

LUVOIR can observe ultraviolet, visible, and near-infrared wavelengths of light and has often been described as a "super-duper Hubble Space Telescope".

Marc Kuchner

Marc Kuchner (born August 7, 1972) is an American astrophysicist, a staff member at NASA's Goddard Space Flight Center (GSFC) known for work on images and imaging of disks and exoplanets. Together with Wesley Traub, he invented the band-limited coronagraph, a design for the proposed Terrestrial Planet Finder (TPF) telescope, also to be used on the James Webb Space Telescope (JWST). He is also known for his novel supercomputer models of planet-disk interactions and for developing the ideas of ocean planets, carbon planets, and Helium planets. Kuchner appears as an expert commentator in the National Geographic television show "Alien Earths" and frequently answers the "Ask Astro" questions in Astronomy Magazine. He currently serves as the principal investigator of the popular citizen science websites Disk Detective and Backyard Worlds.

N. Jeremy Kasdin

N. Jeremy Kasdin is an American astrophysicist pursuing research into the detection and characterization of exoplanetary systems. He is a Professor at Princeton University and Vice Dean of the School of Engineering and Applied Sciences. He is a pioneer of the starshade technique for suppressing starlight to enable the direct detection of Earth-like planets around nearby stars. He is also a recognized authority on orbital dynamics and optimal estimation of physical state, and co-authored the book "Engineering Dynamics: A Comprehensive Introduction". His earlier work included involvement with NASA's Terrestrial Planet Finder mission, a mission studied in the 2000s; an innovative concept for a planet-finding telescope with an unusual pupil, and Gravity Probe B. Kasdin has also been involved with developing a means of tracking birds or other migratory animals anywhere in the world.He is currently the leader of the coronagraph science (the Adjutant Scientist) for NASA's WFIRST mission. Kasdin's work on shaped pupil coronagraphy, one of the techniques being developed for WFIRST, has demonstrated high contrast imaging over a restricted field of view near a bright object such as a star.

Nuller

A nuller is an optical tool used to block a strong source so that fainter signals near that source can be observed. An example of a nuller is being employed on the Keck Interferometer. This causes the light from a star to destructively interfere, effectively cancelling the star's image. As a result, the faint light from a ring of dust orbiting the star can then be detected. This project is part of a scientific effort to detect and observe nearby planets.

Occulting disk

An occulting disk is a small disk used in a telescope to block the view of a bright object in order to allow observation of a fainter one. The coronagraph, at its simplest, is an occulting disk in the focal plane of a telescope, or in front of the entrance aperture, that blocks out the image of the solar disk, so that the corona can be seen. Starshade is one designed to fly in formation with a space telescope to image exoplanets.

Project 1640

Project 1640 is a high contrast imaging project at Palomar Observatory. It seeks to image brown dwarfs and Jupiter-sized planets around nearby stars.Rebecca Oppenheimer, associate curator and chair of the Astrophysics Department at the American Museum of Natural History is the principal investigator for the project.

Solar and Heliospheric Observatory

The Solar and Heliospheric Observatory (SOHO) is a spacecraft built by a European industrial consortium led by Matra Marconi Space (now Astrium) that was launched on a Lockheed Martin Atlas II AS launch vehicle on December 2, 1995 to study the Sun. SOHO has also discovered over 3,000 comets. It began normal operations in May 1996. It is a joint project of international cooperation between the European Space Agency (ESA) and NASA. Originally planned as a two-year mission, SOHO continues to operate after over 20 years in space: the mission is extended until the end of 2020 with a likely extension until 2022.In addition to its scientific mission, it is the main source of near-real-time solar data for space weather prediction. Along with the GGS Wind, Advanced Composition Explorer (ACE) and DSCOVR, SOHO is one of four spacecraft in the vicinity of the Earth–Sun L1 point, a point of gravitational balance located approximately 0.99 astronomical unit (AU)s from the Sun and 0.01 AU from the Earth. In addition to its scientific contributions, SOHO is distinguished by being the first three-axis-stabilized spacecraft to use its reaction wheels as a kind of virtual gyroscope; the technique was adopted after an on-board emergency in 1998 that nearly resulted in the loss of the spacecraft.

Solwind

P78-1 or Solwind was a United States satellite launched aboard an Atlas F rocket from Vandenberg Air Force Base in California on February 24, 1979. The satellite operated until it was destroyed in orbit on September 13, 1985 to test the ASM-135 ASAT anti-satellite missile.

Subaru Telescope

Subaru Telescope (すばる望遠鏡, Subaru Bōenkyō) is the 8.2-meter (320 in) flagship telescope of the National Astronomical Observatory of Japan, located at the Mauna Kea Observatory on Hawaii. It is named after the open star cluster known in English as the Pleiades. It had the largest monolithic primary mirror in the world from its commission until 2005.

Terrestrial Planet Finder

The Terrestrial Planet Finder (TPF) was a proposed project by NASA to construct a system of space telescopes for detecting extrasolar terrestrial planets. TPF was postponed several times and finally cancelled in 2011. There were actually two telescope systems under consideration, the TPF-I, which had several small telescopes, and TPF-C, which used one large telescope.

Vortex coronagraph

A vortex coronagraph is a type of optical instrument which enables the imaging of very faint objects near very bright objects that would normally be obscured by glare. For example, extrasolar planets near their host star as seen from Earth or space telescopes in Earth's solar system. It is a type of coronagraph.

In 2005 a paper described a method for astronomy, by which the light of a parent star could be blocked, while keeping the light from nearby but dimmer exoplanets (or whatever the companion is).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 NASAs Jet Propulsion Laboratory demonstrated that a 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 5 meter Hale Telescope.

Vortex coronographs have been used in conjunction with adaptive optics for astronomy.A vortex coronograph was used on the Keck Observatory by 2017. The VC was installed on infrared camera at Keck, and allowed bodies to be view 2-3 times closer to a parent star than before. HIP 79124 B was imaged at a distance of 23 Astronomical units from its host star with a vortex coronograph on the Keck telescope, and it was called a brown dwarf. Next generation VC are able to dim multiple sources of light. It will be thus possible to use the to image planets around multi-star systems.

Wide Field Infrared Survey Telescope

The Wide Field Infrared Survey Telescope (WFIRST) is a NASA infrared space observatory currently under development. WFIRST was recommended in 2010 by United States National Research Council Decadal Survey committee as the top priority for the next decade of astronomy. On February 17, 2016, WFIRST was approved for development and launch.WFIRST is based on an existing 2.4 m wide field-of-view telescope and will carry two scientific instruments. The Wide-Field Instrument is a 288-megapixel multi-band near-infrared camera, providing a sharpness of images comparable to that achieved by the Hubble Space Telescope (HST) over a 0.28 square degree field of view, 100 times larger than that of the HST. The Coronagraphic Instrument is a high-contrast, small field-of-view camera and spectrometer covering visible and near-infrared wavelengths using novel starlight-suppression technology.

The design of WFIRST is based on one of the proposed designs for the Joint Dark Energy Mission between NASA and DOE. WFIRST adds some extra capabilities to the original JDEM proposal, including a search for extra-solar planets using gravitational microlensing. In its present incarnation (2015), a large fraction of its primary mission will be focused on probing the expansion history of the Universe and the growth of cosmic structure with multiple methods in overlapping redshift ranges, with the goal of precisely measuring the effects of dark energy, the consistency of general relativity, and the curvature of spacetime.

On February 12, 2018, development on the WFIRST mission was proposed to be terminated in the President's FY19 budget request, due to a reduction in the overall NASA astrophysics budget and higher priorities elsewhere in the agency. In March 2018 Congress approved funding to continue making progress on WFIRST until at least September 30, 2018, in a bill stating that Congress "rejects the cancellation of scientific priorities recommended by the National Academy of Sciences decadal survey process". In testimony before Congress in July 2018, NASA administrator Jim Bridenstine proposed slowing down the development of WFIRST in order to accommodate a cost increase in the James Webb Space Telescope, which would result in decreased funding for WFIRST in 2020/2021.

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