When Bernard Lyot created the coronagraphic telescope, he enabled the first extended observations of the Sun’s outer atmosphere without the necessity of a natural solar eclipse.
Although the chromosphere and corona represent only a very small part of the Sun’s total atmosphere, they have long been astronomically interesting yet difficult to access. The subdivisions of the chromosphere and corona together encompass the Sun’s outer atmosphere as inferred from both visual and telescopic observation, photometrically showing only 0.5 10–6 the brightness of the total Sun. During the total stage of a solar eclipse, the Sun’s corona appears as an asymmetric halo, the apparent brightness of which generally decreases from the Sun’s limb (outer edge of the disk) outward. With an overall increase in solar activity every eleven years, additional radial streamers, or rays, can appear outward from the corona. In a period of minimal solar activity, there is simply a diffuse distribution of visible radiation, with minimum intensity at the solar poles and maximum intensity at the equator. The corona’s shape also shows minimum ellipticity every three years prior to a sunspot cycle maximum. Normally, the much greater intensity of the halo, or aureole, of scattered light surrounding the Sun’s disk hides the corona. Observed visually at sea level, the average sky further scatters coronal light to about one one-thousandth of this amount. Only rare locations, such as high mountaintops, offer the chance to observe the corona unaided even during an eclipse.
Research questions concerning the origins and behavior of the corona are important for stellar as well as solar astrophysics. From a planetary solar system perspective, the corona is effectively a transition zone between the solar surface and the interplanetary medium (for example, solar wind). From an astrophysical viewpoint, the conditions that exist in the solar corona are of great interest in determining the laws governing magnetohydrodynamic gases, spectral emissions from highly ionized atoms, and the like. Some of the important questions about the corona concern the temperature implications of the greatly increased spectral emissions above the solar limb and whether the chromosphere and corona are wholly or partly a collection of turbulent gas streams or a more uniform atmosphere.
Until about 1930, the solar corona had been observed astronomically only during total eclipses of the Sun, meaning only a few minutes per year of measurements, often under hurried and less than optimum conditions, with a total observation time from 1630 to 1930 of less than one hour. Spectroheliographs are special solar telescopes that can make observations at any wavelength, but they are slow and unsuited to accurate observations of short-lived eclipse and flare phenomena. Various attempts by astronomers such as Alexander Hale to take spectroheliographs to mountaintops for coronal observation were failures. The first recorded spectrum of the corona was recorded by Georges Rayet
As scientists eventually realized, ordinary reflector or refractor telescopes further scattered solar light from the already weakened and scattered visual appearance. The details of designing and building a special lens and some kind of shielding disk for noneclipse coronal observation required reduction of unwanted scattered light by a factor of more than 100,000. In response to these requirements, Bernard Lyot of the Meudon Observatory conceived, and in 1930 designed and constructed, a new optical telescope in which the diffusion of light inside the lens and telescope tube was minimized to these standards. In Lyot’s original coronagraph, the Sun’s disk is eclipsed artificially, or occulted, by a polished metal cone. The direct image of the remaining (corona) Sun is formed on a black screen slightly larger than the disk’s image. The shiny cone also reflects back the Sun’s light to the side, preventing it from falling back on the main objective lens. Nevertheless, it was further necessary to prevent the solar light diffracted by the main objective from getting into the second, middle objective lens. Lyot accomplished this by placing an aperture stop at the point where the light from the main lens is imaged by the final field lens. A further refinement was to place another opaque disk at the center of the second objective to catch and multiply reflected light from the primary lens and from the occulting cone.
In this chromatically aberrant design, the occulting disk is in focus only for single wavelengths. It is possible to compensate for the overcorrection of the secondary lens so that the final image is aberration-free. To minimize scattering from even microscopic defects of the tube and dust particles, all inner surfaces were smeared with thick oil, thus reducing thermal air currents. The coronagraph requires special care and environmental conditions. All lenses must be kept as clean as possible at all times from dust and aerosol particles. Atmospheric scattering, as noted, is a severe limit, but this is reduced notably for altitudes above a few kilometers, thus most coronal observatories have been constructed at heights above 1,800 meters (about 5,900 feet) and generally above 3,000 meters (9,840 feet) if possible. Winter frequently provides the best observing times; Lyot discovered optimal conditions immediately after heavy snowfalls had cleared the air of atmospheric dust particles.
Wider use of the coronagraph, from 1932 on, provided vastly increased opportunities for coronal observation. Because it is not limited by the brevity of a solar eclipse, Lyot’s coronagraph permitted far more precise measurements of the comparative intensities of different spectral lines vital to astrophysical theories of stellar composition and dynamics. Observations using Lyot’s coronagraph—supplementing observations made during eclipses between 1893 and 1936—showed that the maximum equatorial elongation of the corona occurs 0.7 year before the minimum of solar prominences at higher solar latitudes. Coronal “rays” were shown also to be gas bodies of higher temperature forming above solar faculae, analogous to auroral polar lights insofar as both take the shape of the prevailing magnetic lines of force. Coronagraphic measurements permitted the first rigorous estimates of the temperature, lifetime, luminosity, and characteristic length and height of solar prominences. On the basis of coronagraph and eclipse data, Bengt Edlén
Despite his success, Lyot was not satisfied with his coronagraph’s performance, suspecting that there was a need to reduce further or compensate for residual thermal diffusion within the coronagraph itself. Continuing his experiments, Lyot combined his coronagraph with a spectroheliograph,
Lyot’s final innovation was the coronagraphic photometer, which enabled tracing of weak spectral lines even with ordinary refractor telescopes at sea level. Because of the technical difficulties in making the main lens and meeting all environmental conditions, coronagraphs typically are not large instruments. Among the largest are the 40-centimeter (15.7-inch) coronagraphs of the Sacramento Peak Observatory in Sunspot, New Mexico, and the High Altitude Observatory in Boulder, Colorado. Other coronagraphs are located at Pic du Midi, France; Mt. Norikura, Japan; Freiburg, Germany; and Capri, Italy. As of 2004, the largest coronagraph to date was installed at the Maui Space Surveillance System, an observatory on Mount Haleakala in Hawaii.
To avoid atmospheric scatter, brightness, and fluctuations, from May, 1973, to January, 1974, the Apollo Telescope on the Skylab space station included a special white-light coronagraph. From the thermospheric altitudes of an orbiting spacecraft, the sky is completely black overhead, and coronagraphic observations nearly match those of a total eclipse. Skylab recorded more than one thousand hours of coronagraph data, leading to the discovery of the phenomenon of coronal transients: huge shock waves of ionized gas following a flare or prominence that propagate outward through the corona.
Many researchers consider the period from 1930 to 1947 the watershed between the era of fortuitous uncalibrated coronal observations of opportunity and detailed controlled observations. Bernard Lyot succeeded in accomplishing what many predecessors had tried but failed to do: observe coronal details without an eclipse.
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Beer, Arthur, ed. Vistas in Astronomy. New York: Pergamon Press, 1955. Technical but accessible volume presents valuable information on spectroheliograph and coronagraph technologies and results. -
Dyson, Frank, and Richard van der Riet Woolley. Eclipses of the Sun and Moon. Oxford, England: Clarendon Press, 1937. Reports noncoronagraphic observing methods and results. -
Evans, John W., ed. The Solar Corona. New York: Academic Press, 1963. Intermediate advanced-level text gives important summaries of empirical and theoretical understanding of the Sun’s corona. -
Golub, Leon, and Jay M. Pasachoff. The Solar Corona. New York: Cambridge University Press, 1997. Reviews the information gleaned from observations of the solar corona since the mid-twentieth century. Chapter 5 includes discussion of the coronagraph. Intended for advanced students and researchers. -
King, Henry C. The History of the Telescope. 1955. Reprint. Mineola, N.Y.: Dover, 2003. Excellent overall history of telescopes and attachments presents some material on the coronagraph. Includes photographs. -
Stix, Michael. The Sun: An Introduction. 2d ed. New York: Springer-Verlag, 2002. Introductory technical treatment of solar observations and structure. Includes illustrations.
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