[Paper Review] ROBAST: Development of a Non-Sequential Ray-Tracing Simulation Library and its Applications in the Cherenkov Telescope Array

A. Okumura, K. Noda+1 more|arXiv (Cornell University)|Aug 31, 2015

Astrophysics and Cosmic Phenomena8 references3 citations

TL;DR

ROBAST is an open-source, C++-based non-sequential ray-tracing library built on the ROOT geometry framework, designed for high-fidelity optical simulations of cosmic-ray and gamma-ray telescopes. It enables complex simulations of segmented mirrors, aspherical optics, and hexagonal light concentrators—demonstrated in performance studies of three Cherenkov Telescope Array (CTA) telescope designs and optimized LST light concentrators with validated collection efficiency gains.

ABSTRACT

We have developed a non-sequential ray-tracing simulation library, ROot-BAsed Simulator for ray Tracing (ROBAST), which is aimed for wide use in optical simulations of cosmic-ray (CR) and gamma-ray telescopes. The library is written in C++ and fully utilizes the geometry library of the ROOT analysis framework. Despite the importance of optics simulations in CR experiments, no open-source software for ray-tracing simulations that can be widely used existed. To reduce the unnecessary effort demanded when different research groups develop multiple ray-tracing simulators, we have successfully used ROBAST for many years to perform optics simulations for the Cherenkov Telescope Array (CTA). Among the proposed telescope designs for CTA, ROBAST is currently being used for three telescopes: a Schwarzschild--Couder telescope, one of the Schwarzschild--Couder small-sized telescopes, and a large-sized telescope (LST). ROBAST is also used for the simulations and the development of hexagonal light concentrators that has been proposed for the LST focal plane. By fully utilizing the ROOT geometry library with additional ROBAST classes, building complex optics geometries that are typically used in CR experiments and ground-based gamma-ray telescopes is possible. We introduce ROBAST and show several successful applications for CTA.

Motivation & Objective

• To address the lack of a widely reusable, open-source ray-tracing simulation tool for cosmic-ray and gamma-ray telescope optics.
• To reduce redundant development efforts by providing a common, extensible simulation library for diverse research groups.
• To enable accurate simulation of complex optical systems, including segmented mirrors, aspherical surfaces, and multi-reflection light concentrators.
• To support performance evaluation, tolerance analysis, and shadowing studies for next-generation Cherenkov telescopes like those in the CTA.
• To facilitate the design and optimization of hexagonal light concentrators for improved photon collection in large-sized telescope cameras.

Proposed method

• ROBAST is implemented as a C++ library that extends the ROOT geometry framework (libGeom), leveraging its particle tracking engine for optical photon trajectory calculations.
• Non-sequential ray tracing is achieved through custom classes like AOpticsManager, ALens, AMirror, AObscuration, and AFocalSurface, which model optical components and their interactions.
• Specialized geometric classes such as AGeoAsphericDisk, AGeoWinstonConePoly, and AGeoBezierPgon enable modeling of aspherical mirrors, compound parabolic concentrators, and Bézier-curve-based light concentrators.
• The library supports multiple reflections and refractions without requiring predefined surface order, essential for simulating complex, multi-segmented optics.
• Simulations are validated using spot diagrams and collection efficiency comparisons against idealized and real-world concentrator designs.
• ROBAST integrates with CAD models and performs full 3D optical system simulations, including obscuration and reflection effects from telescope structures.

Experimental results

Research questions

• RQ1 How can a unified, open-source ray-tracing library reduce redundant development of similar simulation tools across cosmic-ray and gamma-ray telescope projects?
• RQ2 To what extent can ROBAST accurately model complex optical systems such as segmented aspherical mirrors and multi-reflection light concentrators?
• RQ3 What is the impact of mirror segmentation and telescope structure shadowing on the point spread function and effective collection area in CTA telescope designs?
• RQ4 How does the performance of a hexagonal Okumura cone concentrator compare to conventional Winston cones in terms of photon collection efficiency?
• RQ5 Can ROBAST support the full design and optimization cycle of advanced optical components like those in the LST camera?

Key findings

• ROBAST successfully models the LST's 198-segmented parabolic mirror system, reproducing fine-structure in the off-axis point spread function due to mirror segmentation and structural obscuration.
• The library accurately simulates the optical performance of two Schwarzschild–Couder telescope designs (SCT and GCT), including shadowing from masts and trusses and multi-reflection effects in aspherical optics.
• ROBAST enables the simulation of multiple reflections in hexagonal light concentrators, which is critical for accurate efficiency estimation in complex geometries.
• The hexagonal Okumura cone design shows improved collection efficiency compared to standard hexagonal Winston cones, with performance approaching that of ideal 2D models.
• ROBAST has been used for tolerance analysis and performance evaluation across three CTA telescope designs, demonstrating its robustness and reusability in real-world applications.
• The library has been integrated into the CTA design process, supporting the development of the LST's optimized, anode-sensitivity-aware light concentrator system.