Preliminary tolerance analysis of the coronagraphic instrument METIS for the Solar Orbiter ESA mission

METIS, the Multi Element Telescope for Imaging and Spectroscopy, is the solar coronagraph foreseen for the ESA Solar Orbiter mission. METIS is conceived to image the solar corona from a near-Sun orbit in two different spectral bands: in the HI UV narrow band at 121.6 nm, and in the polarized visible light band (580 - 640 nm). METIS is an externally occulted coronagraph which adopts an "inverted occulted" configuration. The inverted external occulter (IEO) is a small circular aperture after which a small spherical mirror M0 rejects back the disk-light through the IEO, then an annular mirror collects the signal coming from the corona and redirects it toward the telescope secondary mirror. This paper presents the error budget analysis for this new-concept coronagraph configuration, which incorporates two different sub-channels: the UV imaging and the polarimetric visible one. The two sub-channels are sharing the telescope optics, then an interference filter transmits the UV light towards the UV detector, while the visible-light is reflected towards the polarimetric unit. The tolerance analysis is rather complex, in fact not only the optical performance for the two sub-channels has to be maintained simultaneously, but also the positions of the M0 and the occulters (IEO, internal occulter and Lyot stop), which assure the optimal disk light suppression, have to be taken into account as tolerancing parameters. To guarantee the scientific requirements are optimally fulfilled for the two sub-channels, the preliminary results of manufacturing, alignment and stability tolerance analysis for the whole instrument will be described and discussed.