Diffraction effects in a giant saw-toothed edge externally occulted solar coronagraph

Typical diffraction and vignetting effects associated to the use of both externally and internally occulted coronagraphs make the innermost solar corona quite unobserved. However, by increasing the distance between the coronagraph telescope and the external occulter to hundred of meters, it is possible to observe the solar corona down to 1.01 solar radii without vignetting. This is the case of ASPIICS (Association de Satellites Pour l'Imagerie et l'Interférométrie Coronographique Solaire), a mission proposed to ESA for the PROBA3 program for formation flying. ASPIICS foresees two satellites: the external occulter is located on one spacecraft, and the telescope on the other. In this work we present the results obtained by a theoretical analysis of the apodization of the external occulter. This technique allows the reduction of the diffracted light contribution. We have developed a code that can simulates the effects of the giant external occulter (1 m diameter) on the photospheric light, and calculates the intensity of the diffracted light on the coronagraph entrance aperture. It is possible, in this way, to analyze various shapes of the occulter edge. In particular, we have focused our attention in the case of the serrated disc with variable number of teeth of different length. We considered the simple occurrence of a point-like monochromatic source at infinity. The results show that, employing a toothed disc, improvement of the S/N ratio of many order of magnitude (whit respect to a simple circular flat disc) can be obtained.