Development Of An Optical System For A Bifocal Off-axis Reflectance Spectrometer

Off axis three mirror system is one of the most commonly used reflective optical systems,which has the characteristics of large field of view and good image quality.The design and alignment of off-axis three mirror system with perfect imaging has been mature,and long-term research has been carried out at home and abroad,and a lot of research results have been obtained.However,there are some off-axis three mirror systems for special purposes,which are not perfect,and the final imaging results have large wave aberration,which brings difficulties to the system installation and test.For this reason,this paper studies the design,fabrication and alignment of an optical system for a bifocal off-axis reflectance spectrometer and design a compensation alignment method for an imperfect imaging off-axis three mirror system.In this paper,a bifocal off-axis reflectance spectrometer is studied.The optical system is an off-axis three mirror system.The primary mirror is a concave bicurface,the secondary mirror is a spherical surface,and the three mirrors are oblate spheroidal surfaces.It is used in conjunction with the rear optical system and has a designed residual wavefront RMS of 0.903 λ(λ = 0.6328 μ m).The residual wave front is large,and it is difficult to determine the source of the misalignment,which brings difficulties to the direct installation and adjustment detection.This paper first discusses the process of system design and tolerance analysis,and according to the optical structure of the three mirror system,combined with Gaussian optics and aberration theory,the optical structure of the compensation system used in the system detection is derived,and the tolerance analysis of the structure is carried out.The results show that the compensation system can be used for the system adjustment.Then the processing and testing process of mirrors and the final testing results are explained.The aspheric mirrors adopt the compensation testing method.According to the testing method,the compensation system is designed,and the tolerance and ghost image of the design results are analyzed.The final test results show that the surface accuracy RMS of the three mirrors are 0.012 λ,0.01 λ and 0.012 λ respectively.Then the mechanical structure of the whole off-axis three mirror system is discussed.The mechanical structure of the system and the tested optical components are assembled,and the assembled optical structure is retested to ensure that it is within the given tolerance range.The designed compensation system is assembled by centering to ensure the compensation effect.Finally,the compensation system is successfully used to realize the system alignment.In the process of alignment,the interferometer is used for alignment inspection,and the wavefront aberration and Zernike polynomial data are used to guide the alignment process.The final alignment results are as follows: wavefront RMS=0.13647λ,PV=0.89459λ(λ=0.6328μm).