The Advanced Optical and Thermomechanical Design of the JUICE/MAJIS Spectrometer

The Optical Head design of the Moons And Jupiter Imaging Spectrometer (MAJIS), is presented. MAJIS is a compact visible and near-infrared imaging spectrometer covering the spectral range from 0.5 to 5.54 µm split into two channels, designed for the JUpiter ICy moons Explorer (JUICE) mission [1], that will conduct an in-depth study of the Jupiter system, with particular emphasis on Jupiter, Ganymede, Callisto and Europa. The MAJIS optical layout is constituted by a fore optics, slit and collimator shared between the two channels (VIS-NIR from 0.5 to 2.35 µm and IR from 2.25 to 5.54 µm), followed by a dichroic filter that splits the light between the channels, each one endowed with its own grating, objective and detector. The two focal plane arrays have a matrix of 400×508 (pixel×spectel), 36 µm pitch. A flat mirror mounted in a SU (Scan Unit) before the telescope allows scanning the line of sight in a direction perpendicular to the slit, so to achieve either imaging of a fixed target or motion compensation to increase the dwell time when in close approach periods (e.g. during flybys). In addition, a calibration unit is realized to allow both radiometric and spectral calibration of the two channels, using two different light sources (an incandescent lamp and a black body) that illuminate a common diffuser. Mechanical design is based on a single optical bench which is populated on both sides for volume optimization. The bench implements a network of internal ribs plus a continuous peripheral rib to maximize the stiffness/mass ratio. Three bipods in composite material constitutes the interface with the S/C bench. Bipods design is the result of an accurate trade between thermal insulation from the spacecraft and structural stiffness, still minimizing stresses induced on the bench due to the temperature gradient between the Optical Head and the S/C bench. Low Optical Head operative temperature (<140 K) and cryogenic IR detector temperature (<90 K), required for proper operations in the IR spectral range, are achieved through passive cooling. Two large radiators are implemented on top of the Optical Head, which are in view of the cold space. Double layer of SLI and MLI provides the necessary radiative insulation from the surrounding environment.