The optical design of a concentration system for a solar furnace is studied, proposing several possible solutions. The foreseen use of this solar furnace is to test components and methodologies for solar applications. The analysis assesses and compares the optical performances of several possible configurations. The possibility of employing in a solar furnace an array of off-axis mirrors as primary optics is examined comparing simulations with various diameters and different configurations. In particular the paper compares spherical mirrors, parabolic mirrors with axis inclined with respect to the heliostat rays and a paraboloid with axis parallel to the rays arriving from the heliostat. It proposes an optimal solution, with spherical mirrors on a spherical envelope, which is compared to the heliostat-axis paraboloid. Considering realisation tolerances, mirrors positioning, mirrors pointing and solar divergence effects they equivalently concentrate the sunlight on the receiver. •A concentration system for a solar furnace was optically designed and analysed.•The solar furnace will test components and methodologies for solar applications.•An array of off-axis mirrors is used as primary optics in a solar furnace.•Solutions with spherical mirrors, parabolic mirrors or a paraboloid are compared.