The EnVisS (Entire Visible Sky) instrument is one of the payloads of the European Space Agency Comet Interceptor mission. The aim of the mission is the study of a dynamically new comet, i.e. a comet that never travelled through the solar system, or an interstellar object, entering the inner solar system. As the mission three-spacecraft system passes through the comet coma, the EnVisS instrument maps the sky, as viewed from the interior of the comet tail, providing information on the dust properties and distribution. EnVisS is mounted on a spinning spacecraft and the full sky (i.e. 360°x180°) is entirely mapped thanks to a very wide field of view (180°x45°) optical design selected for the EnVisS camera. The paper presents the design of the EnVisS optical head. A fisheye optical layout has been selected because of the required wide field of view (180°x45°). This kind of layout has recently found several applications in Earth remote sensing (3MI instrument on MetOp SG) and in space exploration (SMEI instrument on Coriolis, MARCI on Mars reconnaissance orbiter). The EnVisS optical head provides a high resolved image to be coupled with a COTS detector featuring 2kx2k pixels with pitch 5.5µm. Chromatic aberration is corrected in the waveband 550-800nm, while the distortion has been controlled over the whole field of view to remain below 8% with respect to an Fθ mapping law. Since the camera will be switched on 24 hours before the comet closest encounter, the operative temperature will change during the approaching phase and crossing of the comet’s coma. In the paper, we discuss the solution adopted for reaching these challenging performances for a space-grade design, while at the same time respecting the demanding small allocated volume and mass for the optical and mechanical design. The view expressed herein can in no way be taken to reflect the official opinion of the European Space Agency.
EnVisS (Entire Visible Sky) is a space camera aboard the Comet Interceptor ESA mission. This is the first F-class space mission, a new generation of fast ESA missions, and it is scheduled for launch in 2029. Comet Interceptor aims to study, by means of in situ observations, a dynamically new comet, or interstellar object, that enters the Solar System for the first time. Approaching the comet, three modules will detach: spacecraft A will provide remote sensing and communications, while spacecraft B1 and B2 will cross the coma and fly-by the nucleus. EnVisS is a fish-eye camera with a field of view (FOV) of 180° × 45°. It is mounted on B2, which is spin stabilized; the spin provides the scanning motion for the camera allowing imaging the whole sky (180° × 360°) including the comet. The EnVisS optical head is composed of ten lenses; the collected visible light passes through a three-strip filter assembly before reaching the detector. The central filter strip is a broadband filter, while the sides filter strips are linear polarizers, with the aim of studying the polarization state of the light reflected by both the comet coma and its core. The optical performance of EnVisS has been evaluated through ray tracing analyses. In this paper, the ghost study will be described and ghost images will be shown. This analysis, performed in the ZEMAX OpticStudio®, highlights which optical element causes the most intense ghost images and shows their distribution over the detector.
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