Performance test of a small-scale prototype of Fresnel optics for cosmic ray observation

For optical detection of cosmic ray, the optics should have a wide-field system. And the system doesn't need a severe angular resolution like an astronomical tele- scope, so we can use a refracting optics which mechanical as- sembly may be looser than a reflecting optics. As a refracting optics, Fresnel lenses provide large-aperture and wide-field systems with drastically reduced mass and photon absorp- tion. Such an optical system is adopted as the basic design of EUSO. Now we are developing a small-scale prototype of Fresnel optics, to establish a large-aperture, wide-field re- fracting optics. This optics has two plastic Fresnel lenses, which are 40cm diameter. These are cut on spherical sub- strates and have grooves in both sides. We report two results of optical tests. One is the result of focusing test using vis- ible lasers. The other is the observation result of ˇ Cerenkov