Research On Design Method Of Off-axis Three-mirror Reflective System Based On Structure Constraints

Compared with the traditional refractive system and the coaxial reflective system,the off-axis reflective system has the advantages of wide spectrum,no chromatic aberration,small size and light weight.In recent years,it has been widely used in many fields such as space remote sensing,wide-field wide-band radiation detection,etc.When designing the off-axis reflective system,each mirror has a high degree of freedom in pose and structure.Therefore,the structure design of the off-axis reflective system is very important,and the structure needs to be adjusted to avoid obscuration.There are some asymmetrical aberrations in off-axis reflective systems that are difficult to correct using conventional spherical and aspherical surfaces.Freeform surfaces have a strong ability to correct aberrations and can be used for aberration correction in off-axis reflective systems.In this paper,aiming at the problems of difficult structure adjustment in the design of the off-axis reflection system,the initial structure of the off-axis three-mirror system is constructed by combining the aberration constraints and structural constraints,and freeform surfaces are used for aberration correction.The main work of this paper is as follows:(1)Five types of monochromatic aberrations in reflective systems are analyzed.According to the primary aberration theory,five primary monochromatic aberration coefficients of the quadratic surface-based coaxial three-mirror system are deduced.The influence of five monochromatic aberrations in the system is analyzed,and the aberration constraints for solving the initial structure of the system are obtained.(2)The method of off-axis and the structural constraints of the off-axis three-mirror system are analyzed.The structural constraints of the system are obtained by controlling the distance between the edge rays of the system and the length of the chief ray of the system.Combined with aberration constraints and structural constraints,the evaluation function for the initial structure construction of the off-axis three-mirror system is obtained.The optimal initial structure of the off-axis three-mirror system can be obtained by optimization.(3)The optimization calculation was carried out using the genetic algorithm,and the initial structure of the system was obtained.Using the design method mentioned in the paper,two types of off-axis three-mirror systems,”O” type and ”W” type,were designed.The parameters of the ”O” type off-axis three-mirror system are: focal length405 mm,entrance pupil diameter 90 mm,F-number 4.5,field of view 3?× 3?.The parameters of the ”W” type off-axis three-mirror system are: focal length 100 mm,entrance pupil diameter 30 mm,F-number 3.3,field of view 3?× 3?.The image quality of the two systems was evaluated,and both systems were close to the diffraction limit and the image quality was good.The feasibility of the method in this paper was verified.