Design of Visible-infrared Integrated Wide-width Space Optical System

In recent years， the development direction of space optical cameras in various countries can be summarized as high resolution， large aperture， and multi-spectral. Compared with single-band visible light imaging optical systems， visible-infrared dual-band optical systems have advantages. Infrared light can penetrate certain objects to obtain information such as their internal structure and heat distribution. At the same time， infrared optical systems have stronger concealment. When combined with visible light systems for imaging and detection of space targets， it can not only acquire rich spectral information across different bands but also achieve all-weather， round-the-clock space remote sensing detection and imaging. Space optical systems can be divided into three design types： refractive， catadioptric， and reflective. Refractive systems have no light obstruction and are easy to assemble， allowing optical design for large fields of view； however， it is difficult to achieve lightweight designs for large-aperture optical systems. Catadioptric systems can achieve large-aperture optical designs， but for systems with large fields of view and infrared detection requirements， the few available infrared refractive materials have low transmittance， poor thermal stability， and high prices. Additionally， lens chromatic aberration is difficult to correct， lens aperture sizes are limited， making it challenging to widely apply in space exploration. Reflective systems combine advantages such as large aperture， wide field of view， long focal length， broad wavelength range， and lightweight design. Currently， most visible-infrared integrated optical systems adopt catadioptric designs， requiring multiple optical elements to correct chromatic aberration， and are unable to meet design requirements for long focal length， large aperture， wide field of view， and lightweight simultaneously.This paper adopts a reflective system structure. The initial structure of the system is solved based on paraxial aberration theory， and optimized design is carried out using optical design software， with reasonable field-of-view settings to avoid light obstruction. Considering factors such as aperture， volume， weight， and environmental adaptability of space cameras， the optical system adopts an integrated design scheme of shared aperture with separate fields of view. The visible light and mid-wave infrared bands share the front-end coaxial three-mirror primary and secondary mirrors. At the rear end， the fields of view are split and redirected via two fold mirrors into the visible light imaging path and the mid-wave infrared imaging path， achieving simultaneous dual-band imaging. To minimize the aperture size of the primary mirror， the first image plane is located near the center of the primary mirror. For the visible light subsystem， a coaxial three-mirror structure is used for an off-axis field； for the mid-wave infrared subsystem， the front end consists of the primary and secondary mirrors of the visible light subsystem， and the light path enters the rear three-mirror anastigmatic off-axis structure via a fold mirror. Regarding detector selection， the optical system uses a visible light TDI-CMOS with 12 288×128 pixels， 7 μm pixel size， and three identical detectors arranged in a ‘zigzag’ pattern on the image plane， utilizing only a part of the non-vignetting region of the image plane， achieving a 36 000×128 pixel array and 25.2 km swath width. The mid-wave infrared subsystem uses a large-format cooled infrared detector with 3 000×2 500 pixels and 18 μm pixel size. This detector consists of a glass window， filter， cold stop， and focal plane， with a distance of 90 mm between the focal plane and the cold stop.The working wavelength range of the visible light subsystem is 0.45~0.9 μm， with a focal length of 5 m， an F-number of 10， and a field of view of 0.7°×2.3°. At an orbital position of 500 km， the imaging swath width is greater than 20 km. The working wavelength range of the mid-wave infrared subsystem is 3~5 μm， with a focal length of 1.5 m， a field of view of 0.2°×2°， an F-number of 3， and 100% cold stop efficiency. The two optical systems share the primary and secondary mirrors of a coaxial three-reflector system at the front end， achieving simultaneous dual-band operation through field-of-view segmentation. The entire system uses eight reflective elements. To meet system specifications and correct aberrations， the three mirrors of the front-end three-reflector system use 6th-order aspheric surfaces， while the three mirrors of the mid-wave infrared system adopt freeform surfaces. The imaging quality of the visible and mid-wave infrared subsystems is evaluated using spot diagrams， modulation transfer functions， and distortion assessments. For both subsystems， the spot diagram RMS radii are smaller than the Airy disk radius， the MTF values at the Nyquist frequency are close to their respective diffraction limits， and distortion is controlled within an acceptable range.A reflective optical system integrating visible and mid-wave infrared light was designed based on paraxial optical theory. This study also verifies the manufacturability of freeform surfaces in engineering practice. The maximum deviation between the freeform surfaces and the optimal aspheric surfaces does not exceed 65 μm， meeting current processing standards. A defocus analysis indicates that the system maintains good imaging quality within the depth of focus. Finally， considering current lens fabrication and assembly conditions， and based on tolerance allocations from previous reflective systems， a tolerance analysis was conducted using the system's rear intercept as a compensating quantity. The results show that the system has good tolerance performance. In summary， the designed optical system meets the initial design specifications， does not require consideration of secondary spectral issues from transmissive elements， affords good imaging quality， and possesses practical engineering feasibility.