JANUS (Jupiter Amorum ac Natorum Undique Scrutator) is a high-resolution camera to be flown on board JUICE Spacecraft, devoted to investigate the atmosphere of Jupiter and the surfaces of his icy moons (Europa, Ganymede and Callisto), in the frame of ESA “Cosmic Vision” program. The scientific objectives that JANUS will reach constrained the design of JANUS Optical Head Unit (OHU), and in particular the specific measurement of Ganymede Libration, imposes highly stringent requirement on the Line of Sight (LoS) knowledge of the instrument. The differential thermal environment conditions of the mission orbits, as well as the instrument heat dissipation timelines, induce optical elements translation and rotations that correspond to a Line of Sight variation. During the mission, the LoS can be characterized with a stellar field or single star observation but none direct measurement of its variation can be retrieved during the scientific imaging sessions. To recover the LoS knowledge, a Structural Thermal Optical and Performance Analysis (STOP) is implemented. The optics and the instrument structure are Finit Element Modeled and processed (NASTRAN) imposing the temperature distributions obtained by the OHU Thermal Model (ESATAN-TMS). The obtained thermo-elastic deformations are then considered in the OHU Optical Model (ZEMAX). The resulting LoS and the dominant OHU temperature gradients are finally entangled with a proportionality relation, as well as its direction uncertainty. The indirect estimation of the LoS variation, and its uncertainty, can be establish, thanks to STOP analysis, in real time during operations as function of installed temperature sensor measurements.
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