The CaSSIS imaging system: optical performance overview

Abstract

The Colour and Stereo Surface Imaging System (CaSSIS) is the high-resolution scientific imager on board the European Space Agency’s (ESA) ExoMars Trace Gas Orbiter (TGO) which was launched on 14th March 2016 to Mars. CaSSIS will observe the Martian surface from an altitude of 400 km with an optical system based on a modified TMA telescope (Three Mirrors Anastigmatic configuration) with a 4th powered folding mirror. The camera EPD (Entrance Pupil Diameter) is 135 mm, and the expected focal length is 880 mm, giving an F# 6.5 in the wavelength range of 400- 1100 nm with a distortion designed to be less than 2%. CaSSIS will operate in a “push-frame” mode with a monolithic Filter Strip Assembly (FSA) produced by Optics Balzers Jena GmbH selecting 4 colour bands and integrated on the focal plane by Leonardo-Finmeccanica SpA (under TAS-I responsibility). The detector is a spare of the Simbio-Sys detector of the Italian Space Agency (ASI), developed by Raytheon Vision Systems. It is a 2kx2k hybrid Si-PIN array with a 10 μm pixel pitch. A scale of 4.6 m/px from the nominal orbit is foreseen to produce frames of 9.4 km × 47 km on the Martian surface. The University of Bern was in charge of the full instrument integration as well as the characterization of the focal plane and calibration of the entire instrument. The paper will present an overview of the CaSSIS telescope and FPA optical performance. The preliminary results of on-ground calibration and the first commissioning campaign (April 2016) will be described.