Clouds cover more than 60% of the Earth's surface and are an important factor in the Earth's radiation budget. However, the spatial and temporal distribution and microphysical parameters of clouds are complex and changeable, and the quantification is complicated. Uncertainties in cloud formation, interactions between cloud and radiation, and cloud parameters pose great challenges to the accuracy of general circulation models. Sensors in the visible and infrared spectral ranges have been developed earlier, and the technology is relatively mature. However, since the scale range of ice cloud particles is large, and the corresponding instrument has a short detection wavelength and is sensitive to small ice cloud particles or thin cirrus clouds, it cannot effectively detect ice clouds of large particles with a large scale range. Terahertz (THz) wavelength is close to the particle size of typical ice clouds, and THz wave has strong interactions, which can be used as an effective supplement to visible and infrared sensors, but the ability of THz wave to detect water clouds is insufficient due to the absorption of water vapor. Aiming at the insufficient coverage of cloud particle detection by existing satellite-borne remote-sensing instruments, this paper proposes an optical system design scheme suitable for multi-channel cloud detection spectral imagers with a wide spectrum from visible to THz bands, which can achieve more comprehensive cloud information detection. Moreover, the integrated observation can also provide more sufficient and convenient observation data for the simultaneous retrieval of cloud information in the visible, infrared, and THz bands.
According to the requirements of cloud parameter retrieval, the system channels are set. A total of 10 detection channels are selected, including four visible/near-infrared (VNIR) channels, two short-wave infrared (SWIR) channels, three thermal infrared (TIR) channels, and one THz channel. According to the orbit height (450 km) and pixel size of the detector, the focal length of the system is determined. Through the evaluation of the signal-to-noise ratio, the system aperture is determined. The aperture of the THz band is set as 150 mm, and F number is 3. For visible and infrared bands, the aperture is 42 mm and 68 mm, respectively, and F number is 2. The optical system adopts an off-axis catadioptric structure. In order to separate the THz band from visible and infrared bands, a method of split-field of view is developed. The THz wave is directly imaged by the off-axis three-mirror system, and the visible and infrared bands are imaged again by the rear optical path. The aperture of visible and infrared bands is separately set to solve the problem caused by the large aperture difference relate to the THz band. Finally, tolerance analysis of each subsystem is carried out step by step according to the degree of optical path overlap.
The imager operates in a push-broom imaging mode to acquire 10 channels' spectrum information from visible to THz bands. The swath width is 100 km corresponding to the orbit of 450 km, and the nadir spatial resolution of THz, visible, and infrared bands are 10 km, 75 m, and 100 m, respectively (Table 1). The main optical system adopts a non-re-imaging three-mirror off-axis anastigmat structure [Fig. 2(a)], the root-mean-square radius of point spot of visible and infrared bands is less than 2 μm [Fig. 2(b)], and that of the THz band is less than 7 μm [Fig. 2(c)], which is much smaller than the pixel size. The rear optical path of the VNIR part adopts the off-axis three-mirror structure again [Fig. 3(a)]. The rear optical path of the SWIR adopts a five-piece transmission type [Fig. 4(a)], which is made of silicon or broadband ZnS. The rear optical path of the TIR adopts two materials, namely, germanium and ZnSe, with a total of four lenses [Fig. 5(a)]. The design results show that the modulation transfer function (MTF) of each subsystem is close to the diffraction limit, the spot diagrams are all smaller than the Airy disk, and the image quality is excellent. Finally, all the sub-optical systems are assembled, and the optical paths are folded (Fig. 6). The total length of the entire system is less than 800 mm, and the volume is relatively compact. The tolerance analysis shows that the MTF drop of each channel does not exceed 0.16 at the Nesquit frequency. The tolerance of the entire system is loose, and the allocation is reasonable.
In this paper, an integrated cloud detection optical system with visible, infrared, and THz wide spectrum bands is designed. The THz band adopts the off-axis three-mirror structure, while VNIR, SWIR, and TIR bands use a secondary imaging structure. The aperture is set separately to realize the multi-channel integrated detection imaging with a large optical aperture difference, and the wide band separation is realized by split-field of view. The structure of the whole system is compact, the imaging quality is good, and the allocation of tolerances is reasonable, which meets the design requirements. The system can verify the development of space-borne THz ice cloud detection technology, and the wide spectrum integrated imaging technology is conducive to the realization of channel registration and remote sensing retrieval and application. In addition to cloud detection, this imaging scheme can also provide a certain reference for other broadband imaging systems.
