Coronagraph

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Coronagraph image of the Sun

A coronagraph is a

active galactic nuclei
(AGN).

Invention

The coronagraph was introduced in 1931 by the French astronomer

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 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 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

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 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.[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.

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 article: 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
    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

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.

Aditya-L1

Aditya-L1
Aditya-L1

Aditya-L1 is a coronagraphy spacecraft developed by the

Lagrangian point between Earth and the Sun.[6][7]

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 the Indian Institute of Astrophysics (IIA) and will continuously observe the Sun's corona from the L1 point.[7][8]

The mission has stringent cleanliness protocols, including a ban on the use of perfumes and sprays by scientists and engineers working on the payload, to prevent contamination that could affect the sensitive instruments.[8]

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.[9] 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.[10]

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.[11] 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.

Hale telescope

Up until the year 2010,

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.[12] 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. ^ "SPARTAN 201-3: Coronagraphs". umbra.nascom.nasa.gov. Retrieved 2020-03-30.
  2. S2CID 18095697
    .
  3. PMID 16389814
    .
  4. ^ Optical vortex coronagraph Archived 2006-09-03 at the Wayback Machine
  5. ^ "NICMOS". STScI.edu. Retrieved 2020-03-30.
  6. ^ Explained: Aditya-L1, India's First Solar Mission
  7. ^ a b VELC payload aboard Aditya-L1 will send 1,440 images of sun a day
  8. ^ a b Strict Measures: Scientists, engineers working on Aditya-L1 weren’t allowed to wear perfumes for THIS reason
  9. S2CID 119544105
    .
  10. ^ "Gemini Observatory Board Goes Forward with Extreme Adaptive Optics Coronagraph". www.adaptiveoptics.org. Retrieved 2020-03-30.
  11. ^ "NASA - Hubble Directly Observes a Planet Orbiting Another Star". www.nasa.gov. Retrieved 2020-03-30.
  12. ^ Andrea Thompson (2010-04-14). "New method could image Earth-like planets". msnbc.com. Retrieved 2020-03-30.

External links

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