The traditional coronagraph is a low scatter visible light optical system designed so that the solar image is formed on an occulting disk which blocks the intense light from the photosphere of the sun, allowing the faint coronal emissions to be recorded in the absence of the nawral occultation of a lunar eclipse. The visible coronal radiation imaged comes from three sources: photospheric radiation scattered by coronal electrons (the K corona), photospheric light scattered by dust (the F corona) and the emission from forbidden tranaltions in highly ionized ions (the E corona) such as the "green line" (Fe XIV X 5303 A) and the "red line" (Fe X . 6374 A). These sources are in general weak, and photospheric light scattered in the insirument creates a serious background problem. The advantage of observations at xtJv and EUV wavelengths for studies of the corona and corona/solar wind interface are obvious. Most of the energy emitted by the corona appears in the resonance lines of ions such as Fe VIII -FeXVI in the wavelength interval 170 A < . < 350 A, where the contrast between the brightness of the solar disk and the inner corona is much less extreme (typically a factor of less than 100, compared to iO -108 for the coronal forbidden lines), placing less extreme demands on an occulting disk. Coronal XUV and EUV observations offer a number of advantages including (i) improved temperature and density diagnostics compared to E coronal observations, (ii) the ability to observe the corona and solar wind to a greater height than is possible with the E corona, and (iii) the ability to follow coronal mass ejections and other transient phenomena from the low corona to the far corona with a single instrument. We describe the design and anticipated performance of XUV and EUV coronagraphs using normal incidence multilayer optics, and comment on farultraviolet (FUV) coronal observations.

Date Published: 1 February 1991
PDF: 13 pages
Proc. SPIE 1343, X-Ray/EUV Optics for Astronomy, Microscopy, Polarimetry, and Projection Lithography, (1 February 1991); doi: 10.1117/12.23211

Published in SPIE Proceedings Vol. 1343:
X-Ray/EUV Optics for Astronomy, Microscopy, Polarimetry, and Projection Lithography