Design of a dual-band MWIR/LWIR circular unobscured three-mirror optical system with Zernike polynomial surfaces

This paper discusses the optical design of an uncooled dual-band MWIR/LWIR optical system using a circular unobscured three-mirror system which is particularly suitable for wide spectral range , large aperture and small volume imaging systems. The system is designed at focal length 310mm, F-number 1.55 with field of view 1.77°×1.33°. A coaxial three-mirror system is calculated by the paraxial matrix as a starting point. With the condition that the focal point of each conic mirror is placed to coincide successively, elements in the system are tilted and decentered properly to make the system unobscured and the mirrors are arranged to form a round configuration for compactness. The optical path is folded inside the region surrounded by the mirrors. Zernike polynomial surfaces which are limited to be symmetric about tangential plane are used to correct aberrations and to improve the image quality. The modulation transfer function of this system is above 0.65 in MWIR band and above 0.5 in LWIR band all over the field of view at the Nyquist frequency of 20 line pairs per millimeter. The result shows that the space can be utilized efficiently, the system is compact and image quality is favorable.