Design of aplanatic spherical four-mirror system for infrared imagery and optimization techniques for optical design have been studied. At first, aplanatic conditions for spherical four-mirror system were derived and the resulting simultaneous equations were solved by bisection method. Cassegrain-inverse Cassegrain, Gregory-inverse Cassegrain, Cassegrain-inverse Gregory, and Gregory-inverse Gregory type were investigated and from these systems Cassegrain-inverse Cassegrain and Gregory-inverse Cassegrain system were available for aplanatic systems. The performance of Cassegrain-inverse Cassegrain system was superior to that of Gregory-inverse Cassegrain system.
Three kinds of damped least squares method, additive damping method, multiplicative damping method, and 2nd derivative damping method, have been studied. All of optimized systems by using above three methods showed similar performances and design data. But, the 2nd derivative damping method showed most stable convergency to the minimum point of the merit function.
The surfaces of the Cassegrain-inverse Cassegrain system were replaced with the conic surfaces to improve the performance, and this system was optimized. Among the investigated 8 types of conic-mirror systems, one system showed outstanding performance. Ge corrector and ZnSe corrector were introduced to improve the performance of the Cassegrain-inverse Cassegrain system, and the system using Ge corrector and the system using ZnSe corrector showed nearly same performance.
In this study, five types of infrared imagery systems were obtained. They were Cassegrain-inverse Cassegrain spherical four-mirror system, Gregory-inverse Cassegrain spherical four-mirror system, conic four-mirror system, Cassegrain-inverse Cassegrain system using Ge corrector, and Cassegrain-inverse Cassegrain system using ZnSe corrector. In these systems, the Cassegrain-inverse Cassegrain system using Ge corrector showed the best performance considering the image quality, manufacturing and mass production.