Comet interceptor's EnVisS camera sky mapping function

Entire Visible Sky (EnVisS) 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 dynamically new comet, or an interstellar object, entering the inner solar system. The EnVisS camera is designed to capture the entire sky in some visible wavelength bands while the spacecraft passes through the comet's tail environment. 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. Very wide angle lens, as a fisheye, must be necessarily anamorphic, i. e. its focal length must change along the field of view, in order to fit a finite-size imaging detector. This anamorphic distortion is introduced by the optical designer, depending on the desired applications. Each possible distortion bring along different field of view mapping and this must be taken into account by the scientific/metrological user, because the plate scale is variable along the focal plane. To obtain useful scientific information from fisheye images (astrometry, flux calibration and brightness measurements), a precise determination of the mapping function has to be accurately determined. In this paper we describe the expected distortion map of the EnVisS camera.