Influence of the thermodynamic properties of the Internal Occulter inside the coronagraph on coronal observations

Abstract

To detect good quality coronal spectra images, the continuous optimization of stray light suppression techniques for coronagraphs is required. The internal occulter (IO) serves as the main tool for the Internally Occulted Coronagraph to suppress the direct light from the photosphere layer, and thermal stress displacements with thermodynamic properties will overcover the information of the internal corona. In this paper, a reflective distribution function model is established according to Kirchhoff's principle which is based on a ground-based Lyot coronagraph, the aperture is 200 mm, detection wavelength is 637.4 nm (Fe X) and the work field range is ±1.05-2.0 RS (RS is the solar radius), thus the absorption rate is inverted. The irradiance at different positions received by the ground is simulated, and then the temperature change of the occulter during the time of the strongest radiation is calculated. The thermal stress displacement change of the two materials was analyzed by the finite element method. Comparison of the experiment shows that the displacement variation of the conical bottom plane results in losing 0.34% RS corona information for the 2a12-t6 aluminum alloy, and losing 0.11% RS coronal information for oxygen-free copper. This way provides a new idea for the thermodynamic modeling of the IO and the direct light suppression technology in the coronagraph.