Optimized optical system with a 1.65 m wide-field corrector for the 6.5 m high-performance MUltiplexed Survey Telescope.

The development of modern large-scale spectroscopic survey telescopes responds to the urgent demand for spectral information in astronomical research. Tsinghua University has previously proposed a 6.5 m MUltiplexed Survey Telescope consisting of a Ritchey-Chretien configuration and a 1.8 m multi-element wide-field corrector, achieving excellent performance and world-leading survey efficiency. However, an optimized 1.65 m multi-element corrector with five lenses is proposed to overcome the constraints on glass uniformity and verification in fabrication of the previous corrector design. It maintains outstanding image quality, with the 80% enclosed energy diameter not more than 0.559 arcsec within 3° FoV over up to a 55° zenith angle. The optimized optical system does not revise the working mode of the ADC or the curvature of the primary mirror while ensuring the reasonability and accuracy of manufacturing of large corrector elements. It provides a more feasible reference optical design for the MUltiplexed Survey Telescope in subsequent iterations and communications with manufacturers.

Compact, UAV-mounted hyperspectral imaging system with automatic geometric distortion rectification.

A highly compact hyperspectral imager with an automatic geometric rectification function is developed in this study, which can be mounted on a UAV for ultra-wide range hyperspectral imaging. For better application, the system can provide visible light image transmission and hyperspectral imaging in the real-time mode. A specific design is proposed to allow the visible light camera and hyperspectral camera to share the same telescope optical path, making the system have a high integration level with a total mass of 1.9 kilograms. Thanks to the sharing-optical-path design, the field of view (FOV), frame rate, and spatial resolution are modified the same between the visible light camera and hyperspectral camera. As a result, the geometric rectification is easily performed, and repeated rectifications are eliminated to improve the imaging efficiency. A FOV of 40 degrees in the frame direction and 26 degrees in the flight direction are realized with a focal length of 13mm, providing a large spectral range from 400 nm to 1000 nm and an excellent spectral resolution of 2.5 nm. An automatic geometric rectification workflow is presented and verified in experiments, which can improve the geometric rectification of hyperspectral images in the presence of low-quality UAV navigation data through the incorporation of frame images. Experimental results show that the relative accuracy of geometric rectification is less than 2 pixels when applying the algorithm to our system dataset.

Underwater hyperspectral imaging system using a prism-grating-prism structure.

An underwater hyperspectral imager (UHI) detection system is designed and developed, and a push-broom scan imaging mode was used. The telescope system adopts an image-side telecentric design with 22 mm focal length and 11∘×11∘ field of view. A prism-grating-prism structure was used as the spectral unit of spectrometer system to reduce spectral line bending. The UHI system reaches 5 nm spectral resolution in spectral range from 400 nm to 800 nm, and its modulation transfer function is greater than 0.62 at characteristic frequency in spectral range. An underwater hyperspectral detection experiment was carried out with an electric turntable, and the UHI system is of good imaging quality and spectral resolution, being able to distinguish various underwater targets.

Spaceborne limb hyperspectral imager for ozone profile detection.

A spaceborne limb hyperspectral imager for ozone detection is designed and developed. The hyperspectral imager can provide the limb hyperspectral radiances images with wide-band and large dynamic range. It is composed of an off-axis parabolic telescope and prism dispersive off-axis aspheric spectrometer, and large dynamic range detection can be realized by using a band-attenuation filter. The spectral range is from 280nm to 1000nm, the field of view is 2.4° (limb vertical direction) × 0.02° (horizontal direction), and the focal length is 69mm. The design results meet the requirements of image quality and have the characteristics of small volume and light weight, thereby making it especially suitable for the application of space remote sensing unlike existing methods that utilize complicated scanning mirror and multiple color separators. The limb hyperspectral imager is measured and calibrated on ground. It detected limb hyperspectral radiances on Tiangong-2 spacecraft of China.