Design of a wide-angle infrared visible viewing system using reflective optics on EXL-50U spherical torus

Abstract A wide-angle integrated infrared-visible imaging system based on reflective optics has been developed on the EXL-50U spherical tokamak. The system enables simultaneous observation of the full poloidal cross-section during plasma discharges through a shared optical path, incorporating both a high-resolution visible camera and a mid-wave infrared camera. To achieve a wide field of view while conserving limited mid-plane window resources, a compact mechanical iris shutter was developed for this system. A dedicated software tool was implemented to support the shutter’s universal parametric optimization, allowing for rapid evaluation of blade configurations and motion trajectories. The final shutter design achieves complete (100%) coverage of the optical aperture and has demonstrated reliable performance under demanding conditions, including high-temperature baking cycles and fueling operations. System calibration was conducted using a simplified in-situ method during vacuum chamber baking. By comparing temperature measurements with those from a calibrated reference infrared camera, the overall optical transmittance was quantitatively determined.