First light of Cassis: The stereo surface imaging system onboard the exomars TGO

L. Gambicorti, D. Piazza, A. Pommerol, V. Roloff, M. Gerber, R. Ziethe, M. R. El-Maarry, T. Weigel, M. Johnson, D. Vernani, E. Pelo, V. Da Deppo, G. Cremonese, I. Ficai Veltroni, N. Thomas

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

2 Scopus citations


The Colour and Stereo Surface Imaging System (CaSSIS) camera was launched on 14 March 2016 onboard the ExoMars Trace Gas Orbiter (TGO) and it is currently in cruise to Mars. The CaSSIS high resolution optical system is based on a TMA telescope (Three Mirrors Anastigmatic configuration) with a 4th powered folding mirror compacting the CFRP (Carbon Fiber Reinforced Polymer) structure. The camera EPD (Entrance Pupil Diameter) is 135 mm and the focal length is 880 mm, giving an F# 6.5 system; the wavelength range covered by the instrument is 400-1100 nm. The optical system is designed to have distortion of less than 2%, and a worst case Modulation Transfer Function (MTF) of 0.3 at the detector Nyquist spatial frequency (i.e. 50 lp/mm). The Focal Plane Assembly (FPA), including the detector, is a spare from the Simbio-Sys instrument of the Italian Space Agency (ASI). Simbio-Sys will fly on ESA's BepiColombo mission to Mercury in 2018. The detector, developed by Raytheon Vision Systems, is a 2k×2k hybrid Si-PIN array with 10 μm-pixel pitch. The detector allows snap shot operation at a read-out rate of 5 Mpx/s with 14-bit resolution. CaSSIS will operate in a push-frame mode with a Filter Strip Assembly (FSA), placed directly above the detector sensitive area, selecting 4 colour bands. The scale at a slant angle of 4.6 m/px from the nominal orbit is foreseen to produce frames of 9.4 km × 6.3 km on the Martian surface, and covering a Field of View (FoV) of 1.33° cross track × 0.88° along track. The University of Bern was in charge of the full instrument integration as well as the characterisation of the focal plane of CaSSIS. The paper will present an overview of CaSSIS and the optical performance of the telescope and the FPA. The preliminary results of the on-ground calibration campaign and the first light obtained during the commissioning and pointing campaign (April 2016) will be described in detail. The instrument is acquiring images with an average Point Spread Function at Full-Width-Half-Maximum (PSF FWHM) of < 1.5 px, as expected.

Original languageBritish English
Title of host publicationInternational Conference on Space Optics, ICSO 2016
EditorsBruno Cugny, Nikos Karafolas, Zoran Sodnik
ISBN (Electronic)9781510616134
StatePublished - 25 Sep 2017
EventInternational Conference on Space Optics, ICSO 2016 - Biarritz, France
Duration: 18 Oct 201621 Oct 2016

Publication series

NameProceedings of SPIE - The International Society for Optical Engineering
ISSN (Print)0277-786X
ISSN (Electronic)1996-756X


ConferenceInternational Conference on Space Optics, ICSO 2016


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