  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.
