Greenhouse gases have a significant impact on the global climate and ecological environment, and are considered one of the primary contributors to global warming. In particular, anthropogenic greenhouse gas emissions have accelerated climate change, with long-term effects on biodiversity, sea-level rise, and extreme weather events. Point source regions emission from power plants, industrial facilities and waste treatment plants are the main sources of carbon emissions from human activities. Accurate monitoring of these sources is important for obtaining high-precision greenhouse gas emission data, evaluating carbon emissions and formulating rational emission reduction policies. The satellite imaging spectrometer is an important optical payload for greenhouse gas emissions monitoring. In order to realize accurate detection of greenhouse gas emissions from point sources, the point source detection payload must exhibit both high spatial resolution and high detection accuracy. This paper presents the design of a high spatial resolution, wide swath, lightweight and compact greenhouse gas point source monitoring payload. The designed imaging spectrometer demonstrates superior imaging quality and compact structural configuration, facilitates straightforward assembly and adjustment, and meets all designated performance requirements.

A wide-swath, compact imaging spectrometer with high spatial and spectral resolution is proposed to meet the monitoring requirements of greenhouse gases in point-source regions. Firstly, according to the detection needs of greenhouse gases, the specifications of the imaging spectrometer are analyzed, and the working band, spectral resolution, signal-to-noise ratio and system F-number of the imaging spectrometer are determined, and the design parameters of the imaging spectrometer are calculated. Then, combined with the system design specifications, the structure selection of the spectroscopic system is carried out, and a symmetric double off-axis three-mirror structure is proposed on the basis of analyzing and comparing the three structures of Chrisp-Offner, Littrow-Offner, and Reflective Triplet. Aberrations of this configuration is examined using wavefront aberration theory, and an initial design incorporating aspheric surfaces is established. In order to further balance and eliminate the system aberration, improve the image quality and reduce the volume, freeform surfaces are introduced to increase the degrees of freedom for optimization. Finally, the design result of the symmetric double off-axis three-mirror structure based on freeform surfaces is presented, along with the evaluation of the imaging quality.

Based on the results of gas retrieval simulation analysis, a symmetric double off-axis three-mirror structure is proposed to meet the design requirements of high spectral resolving power, large spectral dispersion width, and a large numerical aperture. The structure adopts planar transmission grating spectroscopy, and the collimating lens and imaging lens both adopt off-axis three-mirror structure which are symmetrical about the grating (Fig. 10). The aperture stop of the system is set on the planar transmission grating. The symmetric double off-axis three-mirror structure can overcome the problem of the object plane and image plane being too close to each other at large numerical apertures, which is conducive to the suppression of stray light in the system. The final design result of the system shows that the symmetric double off-axis three-mirror structure based on the freeform surface has a size of 310 mm×240 mm×125 mm, the modulation transfer function at Nyquist frequency is close to the diffraction limit, and the spectral bending and color aberration are less than 0.35 pixel size, delivering exceptional imaging quality, compact structural layout, and straightforward assembly and adjustment capabilities, fulfilling all design specifications (Figs. 11?14).

This paper presents the design of a light and compact imaging spectrometer with high spatial resolution for monitoring of greenhouse gas point source regions, based on the results of a specification analysis. In order to overcome the problem that the object plane and image plane are too close to each other caused by the large numerical aperture, a symmetric double off-axis three-mirror structure with plane transmission grating is proposed. The introduction of freeform surfaces significantly improved the imaging quality and effectively corrected aberrations, achieving a lightweight and compact system. The design results indicate that the imaging spectrometer system exhibits excellent imaging quality and compact structure, and has significant implications for enhancing the spatiotemporal resolution of greenhouse gas emission monitoring and for the formulation of effective emission reduction policies.