The Optics Module is formed by two curved double-sided Fresnel lenses and a central lens with a Fresnel surface and a diffractive one for chromatic aberration reduction, all with circular shape and in PolyMethyl Methacrylate (PMMA) material, 10 mm thick.

To fit within the HTV unpressurized opening, the lens maximum diameter is 2.65 m, while the minimun diameter is 1.9 m, owing to cuts on the opposite side.The dimensions of the FoV are ∼ 64◦ and ∼ 45◦ on the major and minor axes, respectively. For these axes, the projected lengths on Earth’s surface are ∼ 500 km and ∼ 330 km, respectively for H= 400 km. The effective solid angle ΩFoV is ∼ 0.85 sr. The wide FoV allows measurement of the entire EAS development from the early stage until it fades out or impacts the Earth.