METIS-ESA solar orbiter mission internal straylight analysis

METIS is the Multi Element Telescope for Imaging and Spectroscopy for the ESA Solar Orbiter. Its target is the solar corona from a near-Sun orbit in two different spectral bands: the HI UV narrow band at 121.6 nm, and the VL visible light band. METIS adopts a novel inverted externally occulted configuration, where the disk light is shielded by an annular occulter, and an annular aspherical mirror M1 collects the signal coming from the corona. After M1 the coronal light passes through an internal occulter and is then reflected by a second annular mirror M2 toward a narrow filter for the 121.6 nm HI line selection. The visible light reflected by the filter is used to feed a visible light (580 – 640 nm) polarimetric channel. The photospheric light passing through the entrance aperture is back-rejected by a spherical rejection mirror. Since the coronal light is enormously fainter than the photospheric one, a very tough suppression is needed for the internal stray light, in particular the requirement for the stray light suppression is more stringent in the VL than in the UV, because the emission of the corona with respect to the disk emission is different in the two cases, and the requirements are a suppression of at least 10-9 times for the VL and a suppression of at least 10-7 times for the UV channel. This paper presents the stray light analysis for this new coronographic configuration. The complexity of the optomechanical design of METIS, combined with the faintness of the coronal light with respect to the solar disk noise, make a standard ray tracing approach not feasible because it is not sufficient to stop at the first generation of scattered rays in order to check the requirements. Also scattered rays down to the fourth generation must be treated as sources of new scattering light, to analyze the required level of accuracy. If used in a standard ray tracing scattering analysis, this approach is absolutely beyond the computational capabilities today available; therefore we opted for a scattering ray generation with a Montecarlo method in which after a father ray hits a surface, only one ray is generated, randomly selected according to the distribution of the transmitted energy. These rays bring with them all the energy that is otherwise distributed between all the rays of second generation, making the model more realistic and avoiding loss of energy due to the rays sampling. The stray light has been studied in function of the mechanical roughness of the surfaces and the obtained results indicate an instrument stray light blocking performance well within the requirements in both channels.