In a satellite-based spectroscopic instrument, the telescope system forms the ground-based image on the slit, which is divided by a dispersive element and finally imaged on a surface detector, with the spectral information represented in one dimension and the spatial information in the other. The spectral and spatial information together is used to quantify the changes in the composition of the atmosphere. In the spectrometer system, the telescope system forms the image of the detection section at the slit, which is spectroscopically separated by the dispersive elements and finally re-imaged by the imaging system on the focal plane array (FPA). The intensity pattern in the spectral direction produced is called the instrumental spectral response function (ISRF). However, changes in the atmosphere and below (clouds, aerosol layer, and ground height) can cause changes in ground albedo or a large number of measured gas emission points. Such changes may lead to inhomogeneous illumination at the slit, which will cause the ISRF distortion and then the detection data deviation. As a result, the detection accuracy of content in atmospheric composition is affected. With the increasing requirements for atmospheric exploration and monitoring, the spectral accuracy of high-resolution spectrometers for Earth observation has increased significantly, but detection data deviation is more obvious when the spectral resolution of the instrument is higher. The concept of slit homogenizer is proposed during the Sentinel-5/UVNS imaging spectrometer development, and it can reduce the date error brought by the spectral signal distortion in the heterogeneous calibration scene.

The main work of this study is to analyze and study the principle of slit homogenizers and the systematic factors and external causes influencing ISRF. In addition, a geometric model and near-field diffraction principle are combined to establish the model structure from the telescope system to the slit homogenizer and from the slit homogenizer to the detector image plane. The equivalent simulation of parallel light is proposed to establish a more concise optical model, which reduces unnecessary calculations while retaining accuracy. Firstly, the rationality of the established model is briefly demonstrated. Based on the optical model, this paper analyzes the optical properties of the slit homogenizer. Through mathematical software and optical software simulation and mathematical analysis, the paper made concrete representations of the homogenizing effect of the slit homogenizer and the influence of the parameters of the slit homogenizer in a homogeneous incoherent scene and makes clearer the working principle and influencing factors of slit homogenizers. Then, the effect of the slit homogenizer is comprehensively evaluated from the entrance pupil scene: Analysis is made on the location of the strong emission point of trace gas and the influence of the coverage area on the ability of the slit homogenizer to maintain ISRF stability. The homogenization effect of the slit homogenizer is studied in different scenes under 25% illumination. The influence of different scenes on the spectrometer system ISRF is quantified. Finally, a method is proposed to improve detection accuracy.

The rationality of the optical model of the slit homogenizer and the ability of the slit homogenizer to calibrate the contrast of different scene cases are demonstrated from the simulation and experimental points of view (Figs. 8 and 11). In the mathematical model, the factors influencing ISRF can be summarized as the spectrograph pupil intensity distribution and the spectrometer system. On the one hand, the object-image relationship of different systems influences the point spread function (PSF), which also has a corresponding effect on the final result of ISRF (Table 1). On the other hand, the spectrograph pupil intensity distribution influences slit illumination, and the homogenization effect of the slit homogenizer based on the established optical model greatly depends on the entrance pupil scene. The difference in cloud and rain positions, the difference in the emission point position, and the size of the coverage area in the entrance pupil scene will affect the ISRF stability (Fig. 13), thus affecting the ability of the slit homogenizer to improve data detection accuracy (Table 2). To improve the ability of the slit homogenizer to calibrate the contrast of the different scene cases, this paper proposes a solution of adjusting the tilt angle of the extension mirror below the slit homogenizer (Fig. 14).

As the new generation of spectroscopic instruments have higher requirements in terms of spatial resolution and radiation accuracy, slit homogenizers are employed to reduce the errors of measurement data from the imaging spectrometers for Earth observation. In this paper, the principle of the slit homogenizer is elaborated using the principle of near-field diffraction. In addition, a simple and reasonable optical model is established, and a more concise and calculable mathematical expression is given. The systematic factors and external causes influencing ISRF are discussed. The ability of the slit homogenizer to keep ISRF stability is found to be limited in the investigation with different positions of emission points and changes in the size of the coverage area, and the error of ISRF may affect data inversion accuracy. Finally, this paper proposes a solution of adjusting the tilt angle of the extension mirror below the slit homogenizer to make up for the deficiency of the slit homogenizer in reducing the influence of extreme scene cases.