The simulation chain for the Terzina Cherenkov telescope on board the NUSES space mission

TL;DR

The paper presents an end-to-end, modular simulation framework for Terzina, a space-based Cherenkov telescope in a $550$ km LEO designed to observe EAS Cherenkov light from the Earth’s limb. It couples primary generation (CPG/nuG) and Cherenkov light propagation (CORSIKA, EASCherSim) with a Geant4 CAD-based detector model (TerzinaG4) and a detailed readout (TerzinaReadoutSim), including adaptive triggering and realistic background/radiation inputs. Optical performance is validated against Zemax using the OpticsLibSim library, with demonstrations based on $60{,}000$ proton showers at $100$ PeV to show photon-density and PSF characteristics. The framework is modular and adaptable to other high-altitude Cherenkov instruments, enabling end-to-end performance studies, background assessments, and design optimization for future space observatories.

Abstract

The Terzina telescope is designed to detect ultra-high energy cosmic rays (UHECRs) and Earth-skimming neutrinos from a 550 km low-Earth orbit (LEO) by observing Cherenkov light emitted by Extensive Air Showers (EAS) in the Earth's atmosphere pointing towards the telescope and in the field of view. In this contribution, a simulation chain for the Terzina telescope on board the NUSES mission will be presented. The chain encompasses all stages of the detection process, from event generation and EAS modelling with CORSIKA and EASCherSim to Geant4-based simulations of the telescope's geometry and optics, followed by modelling of the trigger system and silicon photomultiplier (SiPM) response. The Geant4 module includes the real CAD model of the telescope structure and optical components, with aspherical lenses manually implemented to ensure accurate representation of the optical efficiency and point spread function in Geant4. This comprehensive pipeline, developed using modular C++ code and Python tools for event analysis and reconstruction, produces detailed performance assessments of a telescope operating in a LEO mission but can be adapted for any high altitude Cherenkov telescope, making it a versatile tool for future observatory designs. The possibility of modelling balloons in the atmosphere has also been developed.

Figures (4)

• Figure 1: Schematic overview of the simulation and analysis workflow. The upper part illustrates the generation of true events and background (BKG) contributions, their propagation through detector simulation (TerzinaG4 and TerzinaReadoutSim), and the event selection process. The lower part represents the event reconstruction workflow, including preprocessing and feature extraction from both simulated and observed data, model training and validation for the machine learning model, and the production of science data products using instrument response functions.
• Figure 2: Left: Geometry of the tracks generator. In the 3D-sphere configuration, the generation plane, the closest position of the primaries to Earth, the detector position are shown. Right: Distribution of Cherenkov photons produced by a 100 PeV proton-induced EAS as a function of the primary particle polar angle and the closest position of the protons to Earth.
• Figure 3: Implementation of a Schmidt–Cassegrain optical system in Geant4 (left). Comparison of the optical simulation results showing on-axis, off-axis spot sizes (right).
• Figure 4: Simulation of the FPA response: focal plane hit pattern and corresponding waveforms of a simulated proton event at 300 PeV.