Optical Design and Analysis of the Submillimeter-Wave Instrument on JUICE

The submillimeter-wave instrument on the Jupiter icy moons explorer spacecraft is a passive dual-beam heterodyne radiometer operating in the frequency bands 530-625 and 1080-1275 GHz. The instrument will observe Jupiter's atmosphere as well as the atmosphere and surface properties of its moons. This paper presents the optical design and analysis of the instrument that has been carried out using Gaussian beam mode analysis and physical optics simulations. The optics consists of a 29-cm mechanically steerable off-axis Cassegrain telescope, relay optics, and two feedhorns. Frequency-independent operation of the 600-GHz channel is predicted by the simulations. The 1200-GHz channel shows some frequency dependency because of the selected feedhorn type. The steering of the telescope affects mainly its cross-polarization level. The manufacturing and mounting tolerances, as well as distortion and misalignment by thermoelastic effects, will cause the performance of a real instrument to deviate from that of an ideal one. Physical optics simulations combined with data from finite-element method simulations and measurements of optical surface profiles show that these factors affect, in particular, the instrument pointing. The induced pointing error is up to several arcminutes, whereas the specification is less than 0.5 arcmin. The pointing error can be reduced by adjusting the alignment of the telescope secondary mirror and the feedhorns.