Alignment procedure for detector integration and characterization of the CaSSIS instrument onboard the TGO mission

The Colour and Stereo Surface Imaging System (CaSSIS) is a high-resolution camera for the ESA ExoMars Trace Gas Orbiter mission launched in March 2016. CaSSIS is capable of acquiring color stereo images of features on the surface of Mars to better understand the processes related to trace gas emission. The optical configuration of CaSSIS is based on a three-mirror anastigmatic off-axis imager with a relay mirror; to attain telecentric features and to maintain compact the design, the relay mirror has power. The University of Bern had the task of detector integration and characterization of CaSSIS focal plane. An OGSE (Optical Ground Support Equipment) characterization facility was set up for this purpose. A pinhole, imaged through an off-axis paraboloidal mirror, is used to produce a collimated beam. In this work, the procedures to align the OGSE and to link together the positions of each optical element will be presented. A global Reference System (RS) has been defined using an optical cube placed on the optical bench (OB) and linked to gravity through its X component; this global RS is used to correlate the alignment of the optical components. The main steps to characterize the position of the object to that of the CaSSIS focal plane have been repeated to guide and to verify the operations performed during the alignment procedures. A calculation system has been designed to work on the optical setup and on the detector simultaneously, and to compute online the new position of the focus plane with respect to the detector. Final results will be shown and discussed.