Performance of the EUSO-Balloon optics

EUSO-Balloon is the first balloon pathfinder of the JEM-EUSO (Joint Experiment Missions - Extreme Universe Space Observatory) framework. It is an ultraviolet (UV) telescope designed to test the key technologies and methods necessary for the indirect detection of Ultra High Energy Cosmic Rays (UHECR) by observing from above the fluorescence emission produced by Extensive Air Showers (EAS) in the 290-430 nm band. The instrument was launched on the 24th of August, 2014 by the French space agency (CNES) from the stratospheric balloon base in Timmins, Canada and performed an 8 hour night time flight. One of the key technologies of EUSO-Balloon is the refractive optical system which consists of two large aspherical Fresnel lenses (≈ 1 m2) and aims at focusing the UV light into a detector plane formed by an array of Multi-Anode Photomultiplying tubes. This work presents a description of the optical system, the test bench and methods used to characterize its performance and the global efficiency. The measured performance can be understood through the addition of a semi-empirical diffusion model to the classic ray-tracing model of the optics.