Coronagraphs

The original Lyot coronagraph design (GIF image)

The first objective lens (O1) forms an image of the solar disk and the corona. Great care is taken to select a glass free from inclusions (scattering centers) and the lens is given a highly polished surface. The field lens (F1) forms an image of the objective lens onto the Lyot Stop, where diffraction from the edge of the lens is trapped. and the corona. The second objective (O2) relays the image of the occulting disk and corona onto the detector image plane.

By the mid-nineteenth century it was clearly understood that the solar corona was a part or the Sun, as opposed to an artifact of either the lunar or terrestrial atmosphere. For the next hundred years astronomers attempted to find a method of creating an artificial eclipse of the Sun within the telescope system so as to free observers from the task of locating an appropriate total solar eclipse event in order to see the solar corona. In the late 1890's, the American astronomer G.E. Hale, creator of the Palomar 200-inch telescope, attempted to produce an artificial eclipse of the Sun from a site located on Pike's Peak in Colorado, but unfortunately was not able to produce convincing evidence that his invention was either practical or effective. The technical problem of the production of an artificial eclipse was at last solved by by the French astronomer B. Lyot in about 1932. For his work he was awarded the Copley Medal of the Royal Society.

Lyot recognized that there were multiple reasons that earlier attempts to produce an artificial total eclipse had failed. These included the scattering of light in the earth's atmosphere and the scattering of light within a telescopic objective lens and from the edges of the optical elements. To reduce the effect of atmospheric scattering he chose to locate his instrument at a relatively high altitude site some distance from urban sources of atmospheric pollution. A set of specially arranged lenses and baffles were used to trap out the scattered light generated within the telescope and the optics. His technical solution allowed ground-based astronomers to solution allowed ground-based astronomers to view the lower corona on a daily basis. Features of the Lyot design include a singlet objective lens as the first optical element, an occulting disk used to block the Sun's disk image, and a set of lenses (a field lens and a second objective) which are used to trap the internally scattered light which otherwise would prevent direct observation of the relatively faint corona.

Newkirk's externally occulted coronagraph design (GIF image)

The externally occulted coronagraph uses an extra occulting disk which is placed in front of the first objective lens. A significant reduction in scattered light is achieved by not allowing direct Sunlight to fall onto the first objective. This type of scheme is used for the SPARTAN 201 coronagraph, but the details differ from this simplified schematic diagram. The first objective is a doublet lens used to color correct the final coronal image. The D1 occulting disk is serrated and superpolished to reduce diffraction. The details of the anti-Shuttle glow filter and the polarimeter are also not shown.

In 1941 Walter Orr Roberts, then of the Harvard College Observatory, established a coronagraph station on Freemont Pass in central Colorado, near the site of the Climax molybdenum mine, ironically only a few miles away from the site where Hale had failed years before. The coronagraph was at that time used as part of a piece of military technology in an effort to predict atmospheric conditions affecting communications. An empirical relationship between the ionospheric conditions of the earth and the brightness of the so-called green line corona, a specific emission line produced by iron heated so that thirteen of its outer shell electrons are removed, was used for this task. The story of how this solar-geophysical effect was used to select the date of the Leyte Gulf invasion is recounted in James Mitchner's novel, Space.

In 1966 Gordon Newkirk, of the High Altitude Observatory in Boulder, Colorado, perfected a second type of coronagraph which placed the occulting disk in front of the first objective lens. This design was subsequently used for all U.S. orbital coronagraphs including those of Skylab, OSO-7, P78-1 (a military satellite), the Solar Maximum Mission, and SPARTAN 201.

A set of three coronagraphs, two in the externally occulted configuration, and one of a novel internally occulted design and using a mirror for an objective lens, will be launched into a unique high-altitude orbit on the SOHO spacecraft. These devices are scheduled for launch in the fall of 1995, and it is expected that they will operate from the so-called L1 pont (equal gravitational pull from the earth and the Sun) for several years. This set of instruments will allow observation from the limb of the Sun to a height of over 20 solar radii, since the enromous dynamic range in coronal brightness requires a distinct types of coronagraph configurations for operation at low, intermediate and high altitudes in the solar corona. The advantage of the SOHO orbit is that the spacecraft always faces the Sun, and there is no orbital day or night to interrupt the coronal data collection.

More information on the LASCO coronagraphs on board SOHO is available from the US Naval Research Laboratory.

Text provided by Dr. Richard R. Fisher, NASA Goddard Space Flight Center

Web curator: Joseph B. Gurman

Responsible NASA official: Dr. Richard R. Fisher
Principal Investigator, SPARTAN 201 White Light Coronagraph
Head, Solar Physics Branch
Laboratory for Astronomy and Solar Physics
fisher@c682h.gsfc.nasa.gov
+1 301 286-5682

NASA Goddard Space Flight Center
Greenbelt, MD 20771

Last revised 27 May, 1995 - J.B. Gurman