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.
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 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.
More information on the LASCO coronagraphs on board SOHO is available from the US Naval Research Laboratory.
Responsible NASA official:
Dr. Richard R. Fisher
Principal Investigator, SPARTAN 201 White Light Coronagraph
Head, Solar Physics Branch
Laboratory for Astronomy and Solar Physics
+1 301 286-5682
NASA Goddard Space Flight Center
Greenbelt, MD 20771
Last revised 27 May, 1995 - J.B. Gurman