Study On Anastigmatic Czerny-Turner Spectral Imaging System

Imaging spectrometer is an important optical instrument combining spectrum technology and imaging technology,which is widely used in scientific research,industry,agriculture,military and other fields.The Czerny-Turner structure is an optical structure commonly used in imaging spectral instruments in the market.Compared with other structures,the Czerny-Turner structure has the advantages of high resolution,uniform dispersion,and effectively avoids secondary and multiple diffraction,which is convenient to process and adjust the optical system.With the progress of science and technology,the market has higher requirements for spatial resolution and spectral resolution of imaging spectrometer.However,due to the astigmatism of the Czerny-Turner structure itself,the spatial resolution will be reduced,it is necessary to correct astigmatism.In order to select a reasonable astigmatism elimination structure and improve the traditional Czerny-Turner imaging spectrometer,this paper introduces the relevant theories of the imaging spectrometer,including the principle of grating dispersion,aberration theory and the factors affecting the spectral resolution of the imaging spectrometer.There are four main methods to eliminate the astigmatism of Czerny-Turner structure,including using divergent illumination of grating,replacing spherical mirror with toroidal focusing mirror,adding cylindrical mirror and adding cylindrical lens.In this paper,the principles and astigmatism conditions of the four methods are analyzed.When the divergent illumination of grating is used to correct astigmatism,the first-order conditions are generally used,which ignores the distance from the grating to the mirror in order to simplify the formula.But this paper does not use the first-order condition,and the influence of the distance from the grating to the mirror is considered.According to the object relationship formula,the expressions of the focal length in the tangential plane and the sagittal plane are given respectively,and the values of the relevant parameters are obtained by the simulation of MATLAB,the residual astigmatism is further reduced.In this paper,we use four methods with ZEMAX to improve and optimize the initial Czerny Turner structure in the visible light band,and then carry out the simulation analysis.The slit size of the designed system is 11mm×24μm,the detector size is 1024×1024,and the pixel size is 12μm.In order to improve the signal-to-noise ratio,the method of combining two pixels is adopted.Therefore,the equivalent pixelsize is 24μm,and the magnification of imaging spectrometer is 1:1.The initial focal length of collimator mirror and focusing mirror are 95 mm.The imaging quality of the optimized Czerny-Turner structure is determined by analyzing the modulation transfer function,the spot diagram and the ray fan of the optical system.In addition,the imaging quality,spectral resolution,structure complexity,difficulty of assembly and adjustment,processing technology and cost were compared.The comparison shows that the above four methods can correct astigmatism well in the whole band.The lowest spectral resolution of the optical system optimized by the four methods is 0.65 nm,which is higher than the actual maximum spectral resolution of 0.84 nm.From the optimization effect,the effect of using cylindrical lens is the best.The method of divergent illumination of grating does not introduce new optical elements,and the structure is simpler,the cost is lower,and it’s also easier to install and adjust,which has the highest popularized value in the visible light band.