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.
After the 10th February 2020 launch (04:03 UTC), Solar Orbiter has recently begun its Nominal Mission Phase and is collecting imaging data as never seen before due to its peculiar orbit. The Metis coronagraph produces maps of the linearly polarized visible light corona in the wavelength band 580-640 nm and UV maps in the Lyman alpha H i 121.6 nm line. Metis is a coronagraph characterized by an innovative external occultation system that has a twofold function: reduce the thermal load and remove the diffraction due to the external occulter support. The positions of the entrance pupil (which is called Inverted External Occulter, IEO) and of the actual occulter are switched so that the pupil is the surface facing the solar disk and the occultation is performed by a spherical mirror, M0. M0 is positioned 800 mm behind IEO and reflects the disk light back through the IEO aperture. An Internal Occulter (IO) is conjugated to the IEO with respect to the primary mirror. IO is mounted on a motorized 2-axis stage that allows to perform in-flight fine adjustments to its position. During the on-ground calibration campaign the contribution of the stray light due to the diffraction from the IEO and scattering off the optics was measured. The measurement was carried out by using the OPSys facility in Torino (Italy), which is equipped with a clean environment and a source that simulates the solar disk divergence. A stray light measurement in flight is not trivial due to the presence of the solar corona. Nevertheless, an IO position optimization campaign has been conducted in order to reduce the stray light. A procedure was developed in order to minimize the stray light level on the instrument focal plane. This contribution reports on the procedure and on the results.
This paper illustrates the results of an experiment performed in the frame of the Asiago Observatory Stellar Intensity Interferometry program, aimed to exploit the quantum properties of the photon stream emitted from celestial objects. Data are acquired over two telescopes separated by approximately 4 km in photon-counting mode and analyzed in post-processing. The temporal and spatial correlation function g(2) on the bright star Vega has been successfully measured at zero baseline and at a ~2 km baseline. The result is fully consistent with that expected for a source with the angular diameter of Vega (approximately 3.3 mas).
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