The ocean, covering 71% of the Earth’s surface, serves as the cradle of life and a repository of resources. The development and utilization of ocean resources have significantly altered the ocean environment, which adversely affects the sustainable development. Ocean color, determined by the optical properties of the seawater and the suspended matter, reflects changes in seawater quality and the ocean environment. The imaging spectrometer, integrating optical imaging and spectroscopic technology, is a primary tool for ocean color monitoring. It can not only monitor the coastal environment and the distribution of the elements in the ocean but also can distinguish the composition effectively. With the increasing ocean color application and the urgent requirement of ocean studies, the imaging spectrometer optical system with wide swath and high resolution has important research significance and application value.

To address the ocean color monitoring requirements, we propose and design an optical system of an imaging spectrometer with a wide swath and high resolution. The system operates within a waveband of 0.4?0.9 μm and features a swath width of 140 km, a ground sampling distance of 20 m, and a spectral resolution of 5 nm. The imaging spectrometer is composed of the fore-optics and spectral imaging module. The fore-optics is an off-axis three mirror anastigmat (TMA) system. The spectral imaging module is designed with a unique Offner-like configuration instead of the common Offner configuration. This configuration has similar optical components but with a non-unit magnification. The parameters requirement of the ocean color imaging spectrometer is analyzed and the optical specifications are given out first. Then, the initial structural parameters of the Offner-like configuration calculation method are introduced. Using these calculated parameters, we design and evaluate the optical system for the fore-optics and spectral imaging module. Finally, we compare the optical design results using the conventional Offner configuration, detailing imaging performance characteristics, system sizes, and surface sags to demonstrate the advantages of the proposed system.

In our study, the magnification of the Offner-like configuration is set to be 0.6. At this magnification, the focal length of the fore-optics is 590.75 mm, with an F# of 5. The fore-optics is designed and optimized with even aspherical surfaces. After optimization, the modulation transfer function (MTF) exceeds 0.8 at the Nyquist frequency, and the maximum root mean square (RMS) radius of the spot diagram is less than 2.63 μm (Fig. 4). Given the selected magnification, the image space F# of the spectral imaging module is 3, and the total slit length of is 198.33 mm. Two identical spectral imaging modules, each with a slit length of 99.66mm, are used to achieve the required long slit length. Compared to the traditional Offner configuration, this Offner-like configuration, which still includes a convex spherical surface grating and two mirrors (Fig. 5), offers three advantages compared with the common Offner configuration. First, the magnification is not 1 and can be used as the optimization variable in the optical design. Second, the object and image space F# of the spectral imaging module are no longer equal, and it is possible to reduce the difficulty in designing, manufacturing, and assembling the fore-optics when the selected magnification is less than 1. Third, the incident arm of the spectral imaging module is longer when the magnification is less than 1, and the system size of the imaging spectrometer can be compressed by adding the folding mirror. To enhance imaging performance and reduce the system size, the Zernike freeform surface primary mirror and the tertiary mirror are used. The MTF of the designed spectral imaging module at the Nyquist frequency is larger than 0.8 and the maximum RMS radius of the spot diagram is less than 3.13 μm (Fig. 6). The maximum smile and keystone are about 1.5 μm, which is less than 10% of the pixel size (Table 3).

Our paper presents the design and optimization of an ocean color imaging spectrometer with a wide swath and high resolution, utilizing a unique Offner-like configuration. The dispersion element remains a convex spherical grating, while the spectral imaging module features a non-unit magnification. Compared with the common Offner configuration, the proposed optical system delivers superior imaging quality, reduced size, and enhanced system performance. Our work provides a technical foundation for the development of a wide swath, high-resolution imaging spectrometer for ocean color detection.