Objective In response to the need for high-sensitivity detection of long-range faint and weak targets in the space environment, based on the spontaneous radiation characteristics of the targets, research and design of high-sensitivity optical systems based on long-wave infrared has been carried out and an all-aluminium-alloy coaxial four-inverse long-wave infrared optical system has been developed. The F-number of the system is 2, the total three-dimensional size of the optical structure is 126 mm×88 mm×88 mm, about 91% of the energy of the system is concentrated in the 3×3 image elements of the detector, the aberration is less than 0.5%, the field of view is 3°×3°, the weight of the optical structure is 862 g, and the weight of the whole machine of the infrared detecting system is 2230 g. The optical system adopts the coaxial four-reflector structure, and combines with the commonly used card-type system on the basis of secondary imaging configuration. On the basis of the commonly used card system, combined with the secondary imaging configuration, two mirrors are added on the basis of two reflectors to increase the degree of freedom of the system's aberration optimisation and achieve a larger field of view. The all-reflector structure is conducive to the suppression of thermal radiation noise of the optical machine itself.Reflector and support structure adopt the same kind of aluminium alloy material for the integrated design of the optical system, which has the advantages of wide temperature environment adaptability. The optical system is cleverly designed and has a light and compact structure. The design idea and development method can provide a reference for the optical system of space long-wave infrared detection for similar applications.

Methods The realisation form of the optical system plays a key role in the field of detection systems, and its structural design directly affects the performance and application effect of the system.Different optical structures (e.g., reflective, refractive or composite systems) have different advantages in different application scenarios, and how to choose the appropriate optical system form is a key factor to improve the efficiency and accuracy of the detection system. This paper determines the reflective optical structure form through the analysis of the overall indicators, and analyses the advantages and disadvantages of the commonly used reflector materials, and selects aluminium alloy as the reflector material after comparing a variety of materials. Analyse the principle prototype index to determine the optical design index of the optical system. Using the aberration theory, the initial structure of the coaxial four-reflector optical system is deduced. On the basis of this, the initial structure is determined by combining the design parameters with the requirements of the design indexes, and the initial structure is optimised to obtain the design results and verify the detection capability of the system.

Results and Discussions In order to meet the needs of long-wave infrared optical systems for detecting dark and weak targets at long distances, it is necessary to solve the contradiction between large aperture and light weight and large field-of-view aberration correction. Transmissive structure is ruled out due to the large mass of infrared crystal material, while off-axis triple/quadruple reflector can achieve a large field of view, but the volume is too large. This design adopts the coaxial secondary imaging four-reversal structure: two mirrors are added on the basis of the traditional two-reversal to enhance the degree of freedom of aberration optimisation and realise a 3°×3° field of view; the secondary imaging configuration not only ensures the matching of the cold diaphragm (to enhance the detection sensitivity), but also avoids the noise of the lens' thermal radiation through the all-reflective structure. The system adopts all-aluminium alloy optical-machine integration design, combining light weight and wide temperature adaptability. The measured Noise Equivalent Flux Density (NEFD) shows that the detection distance meets the expected target.

Conclusions In the paper, a coaxial four-reflector structure with secondary imaging is used to design a long-wave infrared detection system that can work in the 7.7-9.3 μm segment, the system F-number is 2, the total three-dimensional size of the system is 126 mm×88 mm×88 mm, and the weight of the optical structure is 862 g. 91% of the energy of the long-wave system is concentrated in the detector's 3×3 pixels, and the aberration is less than 0.5%; the system aberration is well optimised and meets the design requirements. The system aberrations are less than 0.5%; the system has been optimised to meet the design requirements. Through experimental verification, the detection capability of the system reaches the expected goal, which proves the effectiveness of the optical design.