This study presents the optical improvement of a high flux solar simulator (HFSS) with controllable flux-spot capabilities developed for researching solar thermal and thermochemical processes. The HFSS is comprised of seven 2.5 kWel Xenon arc lamps coupled with ellipsoidal reflectors, a servo-controlled attenuator curtain, and three-axes linear test bench. Different attenuators were designed and tested in order to identify the best curtain geometry to improve the HFSS modulation with the lowest possible radiative losses. The optical design improvement was performed with the aid of TracePro, a Monte Carlo ray-tracing software. From simulation results, radiative peak flux from 1700 to 480 kWm-2 from the focal plane to 300 mm further back was estimated without curtains. By using the attenuators, flux levels from 1570 to 92 kWm-2 at the focal plane were also estimated. An experimental validation was achieved with a single lamp-reflector unit obtaining peak flux distributions from 200±20 kWm-2 to 97±9.7 kWm-2 from the focal plane to 300 mm behind. Flux modulation from 170 to 1.5 kWm-2 was also measured at the focal plane using a servo-controlled curtain from fully-open slats (0°) to partially closed (60°). With this attenuator, introduced as the shutter of the system, the use of several lamps or electronic rectifiers is avoided and the radiative flux is modulated with high resolution in an optomechatronical form.