Entire Visible Sky (EnVisS) camera is one of the payload proposed for the ESA selected F-Class mission Comet Interceptor. The main aim of the mission is the study of a dynamic new comet, or an interstellar object, entering the inner solar system for the first time. The Comet Interceptor mission is conceived to be composed of three spacecraft: a parent spacecraft A and two, spacecraft B1 and B2, dedicated to a close and risky fly-by. EnVisS will be mounted on spacecraft B2, which is foreseen to be spin-stabilized. The EnVisS camera is designed to capture the entire sky in some visible wavelength bands while the spacecraft pass through the comet's coma. EnVisS optical head is composed of a fisheye lens with a field of view of 180° x 40° coupled with an imaging detector equipped with both band-pass and polarimetric filters. The design of fisheye lenses requires to take into account some issues typical of very wide-angle lenses. The fundamental origin of the optical problems resides on the entrance pupil shift at large angle, where the paraxial approximation is no more valid: chief rays angles on the object side are not preserved passing through the optics preceding the aperture stop (fore-optics). This effect produces an anamorphic deformation of the image on the focal plane, i.e. the focal length is changing along the elevation angles. Tracing the rays appropriately requires some effort by the designer. It has to be considered that distortion, including anamorphism, is an aberration that does not affect the quality of a point source image, thus it can be present also in well corrected lenses. In this paper the optical design of the mera for the ESA F-class "Comet Interceptor" mission, will be presented together with the initial optical requirements and the final expected optical performances.
EnVisS (Entire Visible Sky) is an all-sky camera specifically designed to fly on the space mission Comet Interceptor. This mission has been selected in June 2019 as the first European Space Agency (ESA) Fast mission, a modest size mission with fast implementation. Comet Interceptor aims to study a dynamically new comet, or interstellar object, and its launch is scheduled in 2029 as a companion to the ARIEL mission. The mission study phase, called Phase 0, has been completed in December 2019, and then the Phase A study had started. Phase A will last for about two years until mission adoption expected in June 2022. The Comet Interceptor mission is conceived to be composed of three spacecraft: spacecraft A devoted to remote sensing science, and the other two, spacecraft B1 and B2, dedicated to a fly-by with the comet. EnVisS will be mounted on spacecraft B2, which is foreseen to be spin-stabilized. The camera is developed with the scientific task to image, in push-frame mode, the full comet coma in different colors. A set of ad-hoc selected broadband filters and polarizers in the visible range will be used to study the full scale distribution of the coma gas and dust species. The camera configuration is a fish-eye lens system with a FoV of about 180°x45°. This paper will describe the preliminary EnVisS optical head design and analysis carried out during the Phase 0 study of the mission.
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