Objective Infrared optical systems used in the military field sometimes need to operate in a wide temperature range of nearly 200 ℃. In this temperature range, the optional types of infrared optical materials are further reduced, and the thermal defocusing phenomenon of the system will be more serious, which leads to the difficulty of the optical system to complete a good non-thermal design. To address this challenge, diffractive elements with unique thermal and achromatic properties are added to the design of optical systems, and a material selection method for secondary imaging systems is proposed. Based on the ideal optical system, the objective lens group and the relay lens group in the optical system are equivalent to the mirror group composed of two single lenses by the equivalent lens theory, and then the material selection of the two mirror groups is completed by the non-thermal map. After in-depth analysis and evaluation, it is determined that the optimal material combination of IRG24 and ZnS for objective lens group is IRG22 and IRG24 for relay mirror group. According to the combination of materials and the distribution of focal power of the ideal optical system, a set of medium-wave infrared optical system is designed. The working wavelength of the system is 3.7-4.8 μm, the field of view is 10°× 8°, the F number is 2, the focal length is 55 mm, the total length of the system is about 115 mm, and the cold stop efficiency reaches 100%. In the range of 20-220 ℃, the modulation transfer function (MTF) of the whole field of view is close to the diffraction limit, and good imaging performance is maintained.

Methods An optical system capable of good imaging in the temperature range of 20 ℃ to 220 ℃ has been established (Fig.4). The optical system adopts the method of secondary imaging. In order to better correct the advanced color difference and achieve the miniaturization of the system, diffractive elements and higher-order aspheric surfaces are added to the optical system. Finally, MTF function is used to evaluate the imaging quality of the system (Fig.9).

Results and Discussions According to the design index of infrared optical system and the design principle of light miniaturization and high energy transmission rate, it is decided to choose refractive secondary imaging system as the initial structure. According to a new material selection method, the material combination of IRG24-ZnS-IRG22 was selected as the glass material of the optical system. Aspheric surfaces were introduced in the system optimization process to ensure good imaging effect and correct advanced aberrations at high temperature. Finally, it can be seen that the MTF function values of the system are close to the diffraction limit in a wide temperature range, which meets the demand of good imaging. After setting a good tolerance value, the quality of the system can still meet the requirements of use.

Conclusions Aiming at the problem that it is difficult to maintain good imaging performance of infrared optical systems operating in a very wide temperature range. Diffraction elements with good performance of heat dissipation and achromatic properties are used to strengthen the heat dissipation ability of the system. In terms of material selection, this paper presents a material selection method for secondary imaging system. Firstly, by constructing an ideal optical system, the parameters of optical elements and the height of paraxial rays are obtained. Then, combining these data with the equivalent lens theory, the objective lens set and the relay lens set are regarded as two single lens systems. On this basis, the material combinations of objective lens group and relay lens group were selected by using non-heat map. Based on this material combination, the optical system is designed. Based on this, the design of the optical system is completed. The optical system consists of only four lenses and one diffractive surface, and the total length of the system is only 115 mm, which meets the miniaturization requirements of the system. The modulation transfer function reaches the diffraction limit and achieves the goal of maintaining good imaging performance in a wide temperature range. Compared with the existing optical systems, this study expands the operating temperature range of the optical system while miniaturizing the system, showing its application potential in near-space hypersonic vehicles. In the next step, the system will be machined to verify its imaging performance in a real-world environment.