Carbon dioxide, methane, ozone, and other gases in the atmosphere can absorb long waves reflected by the ground, and again release infrared radiation to increase the earth temperature. These gases are called greenhouse gases caused by the greenhouse effect and are an important factor causing global warming since industrialization. The world environment changes not only exert a great influence on human life but also lead to the extinction of many species. Therefore, in the face of climate change, greenhouse gas monitoring has become the research focus of various countries, and it is urgent to respond to global warming caused by the increase in greenhouse gas concentration. Greenhouse gas monitoring is the basis for studying the change trend of greenhouse gas concentration, and the composition, nature and intensity of greenhouse gas sources and sinks, and is also the basis for the greenhouse effect and yardstick for formulating emission reduction measures. Achieving the two-carbon goal relies on high-precision and high-resolution traceability of gases such as carbon dioxide and methane. The development of a new instrument for trace gas detection with high precision, low cost, and high timeliness has scientific research significance for carbon emission tracing and detection with high spatial resolution, and the obtained scientific data is vital for formulating carbon neutral strategies. With the continuous progress of the platform technology, higher requirements are put forward for the size of the imaging spectrum system, and light weight and miniaturization have become important development directions of the imaging spectrometers. To solve the above problems, we design a short-wave infrared hyperspectral imaging system with auto-collimation.

In the design method, the image-space telecentric of the telescopic system and the object-space telecentric of the spectroscopic system are ensured to meet the matching conditions of the pupil. The system aberrations are corrected by material matching and lens number increase. Due to the relatively large field of view of the system, the diaphragm is placed in the middle of the mirror group of the telescopic system, which is conducive to correcting the system aberrations. The initial structure obtained from the calculation is set and constrained. After optimization, we invert the collimator group, add the optical splitting element, and adjust the position of the image plane to obtain the optical splitting imaging system. Meanwhile, the system is further optimized to make the imaging quality meet the index requirements. The lens group is reemployed to obtain a spectroscopic system. The picking operation can be carried out, and the optical splitting system can be further optimized under the premise of ensuring the parallel light emission of the single collimator group, which can make the imaging quality meet the index requirements. Symmetric systems with the same structure also have a certain correction degree for aberrations, and the system adopts spherical lens. The optimized telescopic system is connected to the spectroscopic system. The image quality of the system will change after docking. The method of independent design and comprehensive optimization is adopted. The aberrations of the telescopic system compensate for those of the spectral system based on ensuring the sound imaging quality of the individual system. Then the aberrations of the whole system are reduced, and the imaging quality of the whole system is further improved.

An auto-collimation imaging spectrometer is designed, and its structure is shown in Fig. 10. The light beam enters the slit through the telescopic system, and the light emitted from the slit is reflected by the plane mirror to avoid the difficult system layout. The collimator group collimates the light beam, the plane reflection grating is diffracted by the straight light, and the diffracted light passes through the mirror group again for focusing imaging and finally reaches the detector. The groove density of grating is 900 lp/mm and the diffraction order is 1. The working band is 1610-1640 nm, the spectral resolution is 0.1 nm, and the spectral sampling is 0.05 nm. The system design indicators are shown in Table 1. The design results show that the imaging quality meets the requirements. Under the Nyquist frequency of 20 lp/mm, the modulation transfer function (MTF) is better than 0.8, the full field mean square root radius (RMS) is less than 7 μms, and the spectral resolution is better than 0.1 nm, with optical system size better than 460 mm×150 mm×150 mm. Finally, the tolerance analysis of the system is carried out to ensure its feasibility in practical applications. The tolerance MTF is shown in Fig. 13, and the tolerance analysis results are shown in Table 2. The MTF of more than 80% probability value of the whole system is greater than 0.7, and that of more than 99% probability value is greater than 0.58, which meets the practical application requirements of the system.

Light and small atmospheric monitoring loads are more suitable for small carrying platforms and reduce the overall system development cost. We adopt the self-collimating structure to realize the miniaturization design of the system. Based on the grating equation in vector form, the initial structure parameters satisfying the conditions of high spectral resolution and reasonable layout of the system are obtained by deducing its initial structure. Independent design and comprehensive optimization methods are adopted to optimize the whole system and ensure the high imaging quality of the independent system to further improve the imaging quality of the whole system. Finally, the F-number of the light and small short-wave infrared auto-collimation hyperspectral imaging system is less than 3 when the working band is 1610-1640 nm. When the cut-off frequency is 20 lp/mm, MTF is better than 0.8, RMS of each band in each field of view is less than 7 μm, and spectral resolution is better than 0.1 nm, with spectral sampling of 0.05 nm/pixel and the overall size better than 460 mm×150 mm×150 mm, all of which meet the design requirements. We provide a design scheme for light and small imaging spectrometers, and also further basic guarantee and technical support for the future development of miniaturized carrying platforms.