Initial design with confocal conic mirrors for freeform optical imaging systems

Freeformsurfaces that can offer high degrees of freedom for aberration correction have been widely used in various imaging applications. Initial design is particularly critical in the optical design of freeformimaging systems due to the significantly expanded solution space.Here,we present a method to find an initial point for the optical design of four-mirror freeform imaging systems. The method is computationally simple, easily accessible, and theoretically supported. The effectiveness of the method is demonstrated by designing a four-mirror confocal system without field-constant aberration and linear astigmatism.We also generalize the proposed method to four-mirror confocal systems with "double-pass surface."The proposed method can yield a promising starting point for the design of freeformoff-axis imaging systems.