Research Of The Key Techniques Of Large Field Of View Coronagraph

Coronagraph is an instrument observing the corona and coronal mass ejections.Observations of the corona and coronal mass ejections help to study the effects ofthe sun’s magnetic field on the earth. In the mean while, they can monitor the severespace weather events which affecting the earth and the solar-terrestrial space. In thispaper, the Large Field of View Coronagraph is designed for the first time which iscentral occulted and with large field of view. At present, the largest field of view ofthe coronagraph which is central occulted is30R⊙(R⊙is the sun radius). The fieldof view of the Large Field of View Coronagraph is72R⊙in the perigee, and215R⊙in the apogee. This Coronagraph can monitor the corona from the sun to the earth.This paper learns from the international coronagraph manufacture experience.For the Large Field of View Coronagraph in this paper, the main parameters of theoptical system are as follows: the field of view is±20degrees; the pixel resolutionis1.2arcmin; the pixel size is13.5μm; the wavelength is from630nm to730nm;the overall length of system is896mm; the overall length of optical system is356mm; the effective F number is5; the focal length is40mm; and the transfer functionis greater than0.65.The field of view of the coronagraph in this paper is larger than the existingcoronagraph internationally. However the increase of the field greatly increases thedemand of stray light suppression. Therefore, this paper further studies the stray light suppression of the coronagraph. Firstly, the diffraction intensity of the occulter andthe external diaphragm is calculated. Secondly, the theoretical models of thediffracted stray light and the scattered stray light of the coronagraph are constructedin this paper. Diffracted stray light is modeled by the theory of fractional Fouriertransform and Fresnel-Kirchhoff diffraction integral theory. While the scattered straylight is modeled by ABC model. Scattered stray light is mainly caused by the surfaceroughness and dust contamination. Thirdly, the stray light of the coronagraph isclassed to three levels and is suppressed respectively. The first level of the stray lightis the sun light. The second level is the diffracted light from the occulter and externaldiaphragm. The third level is the diffracted light from the objective frame, the lightscattered by the surface roughness and dust contamination, the light scattered by theheat reject mirror and the stray light caused by multiple reflection of the objectivesurface. Finally the overall stray light level of the coronagraph is tested and the straylight can reach10-11B⊙(B⊙is the brightness of the sun).