External occulting screen of high efficiency for out-of-eclipse above-atmosphere coronograph
© Lensky A.V.
The analysis of solar coronal data is of fundamental interest in developing models for solar-terrestrial relationships, including the prediction of geophysical effects. Therefore, the solar corona is becoming more and more interesting for geophysicists. Regular full scale optical observations of the corona are possible only from without the Earth's atmosphere or from its upper part. Presence of a neighbouring bright light source, the sun, 6-10 and more orders stronger than the solar corona offers the main problem of these observations. To record corona under existing conditions, optical device, the white-light coronograph, requires artificial solar eclipse, but simple occulting disk was found inadequate because of too high diffracted sunlight intensity. To reduce instrumental light scattering in space-borne coronograph, more sophisticated external occulting screens have to be used, such as three-disk system or serrated disk. The article deals with the search of screen contour diffracting as little sunlight as possible towards the shaded coronograph. Braced screen is suggested with sector openings bounded by contours which contain smooth curves reversal points but have no points of stationary phase and corner points. By means of asymptotic assessment of boundary diffraction wave integrals explicit expressions for due to the sun illumination of entrance aperture and primary focal plane of the coronograph are obtained in terms of geometrical parameters of the screen. The use of proposed external occulting screen is shown to be liable to reduce illumination of entrance aperture by several orders in comparison with presentday systems and so to liberalize the requirement for light scattering by the main lens what in turn allows to use lenses, mirror objectives included, better corrected for aberrations. Possibility of shelving traditional out-of-eclipse coronograph design, Lyot scheme, is considered, and as a compromise two-channel coronograph version is suggested, containing Lyot channel for inner, nearest to the sun, parts of corona and the other, not by Lyot scheme, for outer corona recording. The article is illustrated by typical examples.