Research On Optical System Design Method Of Long Slit Dual-Band Spaceborne CO₂ Imaging Spectrometer

With the increasingly serious situation of accelerating global warming,to reduce carbon dioxide and other greenhouse gas emissions have become the general consensus of the international community.Satellite remote sensing detection technology is an important means to obtain global carbon emission monitoring and emission reduction assessment because of its characteristics of wide detection range,high real-time and continuous monitoring.As the satellite remote sensing detection tends to develop in the direction of high spectral resolution,high spatial resolution and high sensitivity,the conventional satellite-based CO₂imaging spectrometer can hardly meet the demand of high precision remote sensing detection.At present,the optical imaging system of the spectrometer applied to the payload of carbon satellites has two main problems.On the one hand,the spectral resolution and spatial resolution are mutually constrained,and limited by the incident slit width of the spectrometer system,while the transmitted light flux also affects the level of the signal-to-noise ratio,which determines the ability to obtain high-resolution mapping images.On the other hand,while the spectrometer operates in the weak and weaker wavelength band of atmospheric CO₂absorption,the size of the focal plane detector photosensitive elements and the optical signal utilization,which also affects the accuracy of the spectrometer to obtain more spatial and spectral information of the ground target,and the inversion accuracy of the CO₂gas molecular concentration profile.Therefore,to address the above problems,this paper presents a theoretical analysis and improvement design of the optical system of the spaceborne CO₂imaging spectrometer.The main studies are as follows:Firstly,the research status and development trend of domestic and foreign spaceborne imaging spectrometers are investigated,and the research is carried out for the imaging system of high-resolution and high signal-to-noise ratio spectrometer.The basic principles of imaging spectrometer detection,advantages and disadvantages of different structural designs are analyzed,and according to the design requirements of high-precision remote sensing detection of this topic,the structural design theory of the spectrometer system is studied and analyzed,and the optical system with long slit,dual spectral band,high resolution and high signal-to-noise ratio is designed.The system is designed based on the convex grating Offner imaging spectral design principle of the modified structure,and placed between the slit and the main mirror as well as between the three mirrors and the image plane two curved thick lens,correct the astigmatic aberration and coma,improve the imaging quality of the system.Secondly,for the problem of residual coma and image dispersion in the spectral system generated by the dispersion of the convex grating,based on the geometric optical ray-tracing equation,the tangential relationship at the central wavelength between the meridian ray and the arc-vector ray is used to overcome the above problem,so as to establish a reasonable initial structure and improve the utilization of the incident ray in the whole spectral range.And comprehensively consider the detector image element size,detector and focal plane back intercept control,glass material and material performance limitations,and the actual working environment of the imaging spectrometer,by optimizing the optical system locally and globally through the optical design software,the final design of the dual-incidence slit on-board CO₂imaging spectrometer,which has a dual-incidence slit length of 30 mm,and an operating F-number of 2.5.The system can achieve high accuracy detection with 0.1 nm spectral resolution in the dual spectral range of 1594～1619 nm and 2035～2070 nm,and further improve the signal-to-noise ratio of the spectral system by CMOS digital-domain image element merging.Finally,considering the temperature influence of the optical system in the actual working environment,the operating environment of the optical system and the temperature-induced changes in material,refractive index,and optical focal length are analyzed,and the optical passive heatless compensation technology is used for the heatless design of the system at-30℃～40℃.Analysis of the design error in a range of distribution on the optical system performance,as well as the final image quality of the system under factors such as radius of curvature,lens face shape,and tilt of the mirror set when the optical system is mounted.The design results show that after the reasonable allocation of optical system tolerances,with the optical system transfer function,wave image difference,and point column diagram as the evaluation criteria,the given tolerances meet the actual engineering design requirements and can ensure that the system has good imaging quality.