A lightweight Schmidt space telescope configuration for ultra-high energy cosmic ray detection

One of the most challenging open questions of modern astrophysics and particle physics is the determination of the origins and the production mechanisms of the Ultra-High Energy Cosmic Rays (UHECR), i.e. particles with energy above the Greisen–Zatsepin–Kuzmin (GZK) limit, which is about 5×1019 eV. UHECRs can be studied in two ways: either via direct detection of the secondary particles, i.e. extensive air shower (EAS), produced by UEHRCs interaction with the atmosphere, or by observing during night the track of the UV fluorescence emitted by EAS. The origin direction of the cosmic rays can be therefore determined. While ground-based observatories are already operative, different optical configurations, based mainly on the Schmidt camera layout or double Fresnel lenses concept, can be envisaged for future space-based ones. Both solutions faced in the past technological issues: transmission and resolution at large field angles for Fresnel lenses and weight of the primary mirror for the Schmidt. However, recent advances in the technology of ultra-lightweight, large and deployable active mirrors made the Schmidt camera approach feasible, becoming the preferred option. This work describes a lightweight Schmidt space telescope design for UHECRs detection conceived for a mission intended to orbit at 600 km altitude. The instrument concept is a fast, high-pixelized, large aperture and large Field-of-View (FoV) digital camera, working in the near-UV wavelength range with single photon counting capability. The telescope will record the track of an EAS with a time resolution of 2.5 μs and a spatial resolution of about 0.6 km (corresponding to ~ 4’), thus allowing the determination of energy and direction of the primary particles. The proposed design has about 50° FoV and a 4.2 m entrance pupil diameter. The mirror is 7.5 m in diameter, it is deployable and segmented to fit the diameter of the considered launcher fairing (i.e. Ariane 6.2). The Schmidt corrector plate is a lightweight annular corona. This configuration provides a polychromatic angular resolution less than 4' RMS over the whole FoV with a very fast relative aperture, i.e. F/# 0.7. Thanks to its very large pupil and large FoV, the design could be fit for a space-based observatory, thus enhancing the science achievable with respect to the presently operating ground-based counterparts, such as Telescope Array and Auger. A key advantage of this catadioptric design over the classic all refractive adopted in the past is the higher attainable global throughput. This parameter guarantees to reach and fulfil the required instrument photon collection specifications.