Preliminary internal straylight analysis of the METIS instrument for the Solar Orbiter ESA mission

METIS, the multi element telescope for imaging and spectroscopy, is a solar coronagraph foreseen for the Solar Orbiter mission. METIS is conceived to observe the solar corona from a near-sun orbit in three different spectral bands: in the HeII EUV narrow band at 30.4 nm, in the HI UV narrow band at 121.6 nm, and in the visible light band (500 - 650 nm). The visible light from the corona is ten million times fainter than the light emitted by the solar disk, so a very stringent light suppression design is needed for the visible channel. METIS adopts an "inverted occulted" configuration, where the disk light is shielded by an annular shape occulter, after which an annular aspherical mirror M1 collects the signal coming from the corona. The disk light heading through M1 is back-rejected by a suitable spherical mirror M0. This paper presents the stray light analysis for this new-concept configuration, performed with a ray tracing simulation, to insure the opto-mechanical design grants a stray light level below the limit of 10-9 times the coronal signal intensity. A model of the optics and of the mechanical parts of the telescope has been realized with ASAP (Breault Research TM); by means of a Montecarlo ray tracing, the effect of stray light on VIS and UVEUV channels has been simulated.