ROOT-Based Simulator for Ray Tracing (ROBAST) is a ROOT-based ray-tracing simulation library for reflective optical systems. In very-high-energy cosmic-ray research, large optical telescopes are built to observe the atmospheric Cherenkov light and fluorescence from air showers. ROBAST was developed to allow cosmic-ray researchers to easily simulate such telescopes.
ROBAST was originally developed for simulating the optical system of the Ashra experiment. Fig 1. shows the Ashra optical system built with ROBAST. It is now primarily used for telescope and light-concentrator development for the Cherenkov Telescope Array Observatory (CTAO). As shown in Fig 2., ROBAST can easily simulate complex optical systems such as a Schwarzschild-Couder (SC) design with many segmented mirrors.
CTAO uses six different optical designs. ROBAST has been used in the development of four of them — the Large-Sized Telescope, the Medium-Sized Telescope, the Schwarzschild-Couder Medium-Sized Telescope, and the Schwarzschild-Couder Small-Sized Telescope — making it one of the most widely used optical simulation tools in the cosmic-ray and gamma-ray field.
ROBAST uses the ROOT Geometry library, making it straightforward to construct complex three-dimensional optical systems and trace photons through them. Results can be immediately analyzed and visualized using ROOT classes such as TH2 and TGraph. Fig 3. shows spot diagrams of an ideal Schwarzschild-Couder optical system.
ROBAST can also directly read the Cherenkov photon output files from CORSIKA, the well-known air-shower simulator, enabling end-to-end simulations from air-shower to trigger.
For more details, see the ROBAST website and the publication.