Design Of Off-aixs Three-Mirror Optical System With Free-Form Surface

With the development of space technology,space remote sensing technology to develop in a high-resolution,lightweight and versatile direction.This requires the space optical system has a number of characteristics,such as large aperture,long focal length,wide field-of-view(FOV),compact structure,high image quality and low distortion.Off-axis three-mirror optical system has no chromatic aberration and central obstruction,and is easy to retain large FOV,high image quality and compact package.However,with the ever-growing demands of imaging performance and quality for space optical sensor has brought big challenges to traditional aspheric optical system.Free-form surface optical elements with non-roational symmetric surface,can provide maximum degrees of design freedom,improve the system ablility of balancing hig-order aberations and off-axis aberrations,expand system effective FOV,and make system structure more flexible.However,test of free-form surface has been the choke point of its application and development.CGH(Computer-Generated Hologram,CGH)can generate wave front of any shape through diffraction,its application in the null testig of aspheric mirrors has the advantage of high efficiency and high precision.For the application of high resolution,large FOV space remote sensor,in this article,the optical structure is selected and system parameters are determinated.Then based on the primary aberration theory,the initial structure calculation process of the off-axis three-mirror optical system is specified.After optimization,the optical system whose focal length is2000 mm,the entrance pupuil is 235 mm is obtained.The imaging quality of the system is analyzed,and the tolerance allocation results of the system are verified through the Monte-Carlo method.On this basis,the initial structure of off-axial optical system with spherical secondary is calculated.Then,free-form surface in the form of Zernike polynomial is introduced.Optimization of the system has taken the advantage of the relationship between Zernike polynomial and aberration coefficient.The off-axis three-mirror optical system equiped with a free-form surface primary mirror is obtained,with the same focal length(2000mm)and entrance pupil(235mm).The imaging quality and tolerance of the system are analyzed.Comparison of conventional off-axis three-mirror otpcial system and off-axis three-mirror optical system with free-form surface shows that the latter one has an outstanding optical performance especially in FOV and distortion.For the large aperture off-axis three-mirror optical system with free-form surface,the test method which based on CGH is introduced.The principle,classification and characteristics of CGH are expounded,and the key parameters are analyzed.The test scheme of free-form primary mirror is designed,the phase function of CGH is solved,and the validity of the design is verified by simulation with Zemax.In this articale,a reliable case for the large aperture,long focal length,large FOV space remote sensor is provided,an effective design method for the off-axis three-mirror is set forth,and the detailed design method of free-form surfaces null testing based on CGH is elaborated.This research has a stronger application value.