Research On Imaging Optical System Of Space Target With Large Depth Of Field

The depth of focus or field of a traditional optical system is usually determined by its aperture,diaphragm,focal length and other structure parameters.A larger depth of focus or field enables the system to obtain more information from the objective space and reduce many problems caused by defocusing.In space target imaging,it is required that the target can be clearly imaged at different limited distances,because different distances correspond to different optimal image plane positions,which means that the optical system must have a larger depth of focus;in addition,due to the complex space environment such as vibration and temperature,the structural parameters of the optical system and the position of the image plane will also change.Usually,in order to ensure clear imaging,a focusing mechanism needs to be added to the optical system,which makes the imaging system more complicated.And it's reliability declines.Therefore,the large focal depth imaging system without focusing mechanism has a wide range of needs in many fields such as aerospace.For a long time,expanding the depth of focus technology has been a goal pursued by people.Wavefront coding technology is a novel technology to extend the depth of focus which combines optics and image processing.Wavefront coding is based on the traditional optical system.A specially designed phase mask is inserted into the pupil to modulate the wavefront phase at the pupil so that the optical transfer function and point spread function of the system are within a certain defocus range.The internal change is very small,so as to achieve the purpose of extending the depth of focus.The wavefront coding technology can extend the focal depth of the system to more than one order of magnitude without reducing the amount of light passing and reducing the resolution,so it has extremely high research significance and application value.In this subject,wavefront coding technology is used to research and design a space target imaging optical system that achieves large depth of field and clear imaging of different finite object distance targets without focusing mechanism.At the same time,a gradient prior with first-order and second-order as the main form is proposed,and the objective function is optimized based on the reasonable design of the prior,and finally a good image restoration effect is achieved.It satisfies the large depth of field imaging requirements for objects with varying object distances in the space target imaging optical system,and can restrain the defocus of the optical system caused by temperature changes and vibration,and is verified from the principle and simulation experiments.Based on the analysis of the current research and development status of wavefront coding technology at home and abroad,this paper conducts research from basic theory,design,simulation and image restoration,And a detailed introduction of a space target imaging optical system based on wavefront coding without focusing mechanism and large depth of field.The specific research work of this paper is as follows:First,the basic theory of wavefront coding technology is analyzed from two parts:coding and decoding.The mathematical form of the cubic phase plate is deduced,the defocus stability of the wavefront coding system is verified,and the method and evaluation index for restoring the intermediate image are proposed.Second,research and design a set of space target imaging optical system without focusing mechanism that uses wavefront coding to expand the depth of field.First,a set of initial space target imaging optical system with a focal length of 20 mm and an entrance pupil diameter of 7mm is designed.Secondly,comprehensively considering the system's ability to expand the depth of field and the image restoration quality,the surface parameters of the cubic phase plate are optimized.Finally,the optimized cubic phase plate is used in the initial optical system to expand the depth of focus to 28 times of the initial system and the depth of field to 0.5m?130m.Third,aiming at the deblurring problem of wavefront coding technology and the characteristics of single-spectrum gray-scale imaging of the designed optical system,a gradient prior with first-order and second-order differentials as the main form is proposed,and the objective function is reasonably designed and optimized,and FOPD algorithm is used.Finally,a good restoration of the blurred image in the wavefront coding system is achieved.