Simulation of the EnVisS camera polarizing filters

Abstract

The Entire Visible Sky (EnVisS) camera is one of the instrument aboard the Comet Interceptor (CI) mission designed to perform remote sensing. CI is the first Fast-class mission conceived by the European Space Agency (ESA) and its primary scientific goal is to deepen our understanding of the formation and evolution of comets and, more generally, of the Solar System. To achieve this goal, CI will target a Dynamically New Comet (DNC), and EnVisS will analyze the radiance and polarization properties of the light scattered by the dust particles of its coma. The fish-eye camera operates in the visible wavelength range and is developed to work in push-broom mode, taking images of the entire sky from a point inside the comet coma. One of the key elements of the instrument that will make this study possible is the camera Filter Strip Assembly (FSA). The filter assembly concept was conceived by CNR-IFN of Padova and Leonardo SpA of Campi Bisenzio, Florence, in close collaboration with the chosen filter manufacturer, Materion Balzers Optics (MA, USA). The FSA is composed of three filter strips glued side by side, all working in the range 550–800 nm. The strips have the same operational size and different optical properties. In particular, the central filter is a high transmission broadband filter, while the lateral ones are linear polarizers with polarization axis at 45° from one another. The operating filter structure is quite complex: it foresees a sandwich structure composed of several layers of different materials and an anti-reflection coating on both sides. The unused parts of the assembly will be covered by a dark black coating to minimize scattering and give the desired dimensions of the three filter strips. One of the layers composing the sandwich structure is Moxtek polarizer UBB01A. After experimentally evaluating the optical performance of these filters, a simulation of the NanoWire Grid Polarizers has been performed by means of the COMSOL Multiphysics software, to compare the theoretical and experimental results. This paper begins with a presentation of the CI mission and the EnVisS camera, then the FSA is described in detail. This introductory part is followed by a thorough description of the simulations performed. Lastly, the preliminary results obtained from the analysis of these components are reported and discussed.