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The Extreme Universe Space Observatory (EUSO), onboard the Japanese Experiment Module (JEM) of the International Space Station (ISS), is a project led by Japan, with contributions from the United States, Italy, Germany, France, Spain, Switzerland, Russia, South Korea and Mexico. It is devoted to the detection of ultra high-energy cosmic particles with energies E>7x1019eV, which are revealed through emission in the atmosphere of Cherenkov and fluorescence light in the near-UV region. Current experiments are all ground-based. A big enhancement would rise from space, since a bigger atmospheric target could be monitored. However, since at these high energies the signal is faint and the probability of detection is very low (~1 Km-2 century-1), the optical system must have a large aperture, wide Field of View (FoV) and be necessarily lightweight. This project is the continuation of the EUSO mission, led by ESA, stopped some years ago at the end of phase A, and JEM-EUSO is currently at the end of phase A. For both experiments, a ~2.3 m Entrance Pupil (EP) diameter and a 60° FoV were required to achieve the science goals. However, for the present configuration, the constraint of the maximum stowable dimensions of the JAXA's H-II Transfer Vehicle (HTV) unpressurized cargo area forces the instrument to have maximum transverse dimensions of 2.65 x 1.9 m2. Reflective optics, in the form of a properly designed Schmidt camera, are not yet suitable for this purpose, since these optical requirements would need a large, deployable, primary mirror. The main challenge for designing the current configuration consists in developing an unusual combination of large and lightweight refractive optics: two double-sided curved Fresnel lenses and a central curved Fresnel + diffractive lens, in UV-grade PMMA and/or CYTOP, have been considered. This paper describes the development of such a system, focusing on the possible choices of materials and overall optical design, which is responsibility of the authors. Performances of the latest configurations are also presented.
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