JANUS is a multi-filter optical camera part of the JUICE ESA Mission, that has been launched last April from the French Guiana towards the Giovian system, where it will arrive in 2031. During the design phase of the instrument an extensive Straylight Analysys has been carried on, but after AIV the need to update the analysis on the base of the 'as built' system has become desiderable, to better interpretate the calibration data and prepare for science phase. We here report about this update, covering the rationale of the update, the used methodology and the obtained results.
JANUS (Jovis Amorum Ac Natorum Undique Scrutator) is a high-resolution camera operating in the spectral range 340-1080 nm and designed for the ESA space mission JUICE1 (Jupiter Icy moons Explorer) planned for launch in 2023 and arrival at Jupiter in 2031. The main scientific goal of the mission is the detailed investigation of Jupiter and its Galileian moons: after three years in Jupiter orbit and many fly-bys with the icy moons, JUICE will be the first spacecraft to be inserted in orbit around Ganymede in 2032. During the final stage of the mission, JANUS is expected to provide images of the moon surface with ground sampling up to 7.5 m/pixel in both panchromatic and narrow band spectral ranges. Leonardo Spa is the JANUS prime contractor and is in charge, on behalf of the Italian Space Agency (ASI) and in collaboration with the science team led by Parthenope University and the Italian Institute of Astrophysics (INAF), of developing and integrating the Opto-Mechanical Structure of JANUS Optical Head Unit (OHU). The present paper will discuss the procedure adopted for the integration of the OHU and the results in terms of optical quality of the system in flight conditions. Ensuring a Modulation Transfer Function (MTF) close to the diffraction limit at the Nyquist frequency of 71.4 cy/mm constitutes the main challenge for the telescope integration and sets the maximum acceptable transmitted wavefront error to be at most a few tens of nm over the whole field of view.
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