Out-of-focus point sources image simulation for the Metis solar coronagraph onboard the Solar Orbiter mission

Metis is a multi-wavelength coronagraph onboard the European Space Agency (ESA) Solar Orbiter mission. Thanks to the selected Solar Orbiter mission profile, for the first time the poles of the Sun and the circumsolar region will be seen and studied from a privileged point of view near the Sun (minimum distance 0.28 AU). Metis features an innovative instrument design conceived for simultaneously imaging the visible (580-640 nm) and ultraviolet (Lyman α at 121.6 nm) emission of the solar corona. METIS is an externally occulted coronagraph which adopts an "inverted occulted" configuration. The inverted external occulter (IEO) is a circular aperture after which a spherical mirror M0 rejects back the solar disk light, which exits the instrument through the IEO aperture itself. The passing coronal light is then collected by the METIS telescope. Common to both channels, the Gregorian on-axis telescope is centrally occulted and both the primary and the secondary mirrors have annular shape. The optical and radiometric performance of the telescope is strongly dependent on the huge degree of vignetting presented by the optical design. The internal fields are highly vignetted by M0 and further vignetted by the internal elements, such as the internal occulter and the Lyot stop, furthermore the presence of some spiders, needed to mount the internal elements, are vignetting even more, in some parts of the FoV, the light beams. During the instrument commissioning, in the visible light channel some out-of-focus sources have been imaged while moving in the Metis FoV. At a first glance, the out-of-focus images exhibit a very strange pattern. The pattern can be explained by taking into account the peculiar design of the Metis coronagraph instrument; in fact, the not fully illuminated pupil gives rise to "half moon" shape out-of-focus images with the spiders casting their shadow in different positions. In this work, the ray-tracing simulation results for the out-of-focus images are compared with some of the images taken in flight; some considerations relating the shape and dimension of the acquired images with the distance from Metis of the sources are also given.