Optical design of a cooled mid-wave infrared off-axis three-mirror system with a low F-number and wide field of view.

Infrared imaging systems with a wide detection scale, excellent precision, and high resolution are playing an increasingly important role in many fields. Compared with uncooled infrared detectors, the cooled infrared detectors have the advantages of high precision, long detection distance, and a high signal-to-noise ratio due to the low working temperature. In this paper, a cooled off-axis three-mirror mid-wave infrared (MWIR) optical system with a wide field of view (FOV), long focal length, and large relative aperture is designed. We first establish the initial coaxial three-mirror system based on the third-order Seidel aberration theory and then make the aperture stop offset to eliminate central obscuration. During the process of optimization, the FOV of the system is expanded stepwise, and it is not biased to avoid the extra aberration. In order to increase the optimizing degrees of freedom to improve the aberration elimination ability of the system, we apply the XY polynomial surface on the tertiary mirror (TM). After optimization, we finally obtain a cooled off-axis three-mirror MWIR optical system with a FOV of 5.3∘×4.4∘, a focal length of 500 mm, and a relative aperture of 1/2. The image quality of our system is excellent, and the aperture stop is between the focal plane and the TM, and coaxial with the focal plane, meeting the conditions of matching with the cold stop of the infrared detector. The tolerance analysis is also carried out, and the result shows that the system meets the requirements of practical fabrication and alignment.

Optical Design of Imaging Spectrometer Based on Linear Variable Filter for Nighttime Light Remote Sensing.

Nighttime light remote sensing has unique advantages on reflecting human activities, and thus has been used in many fields including estimating population and GDP, analyzing light pollution and monitoring disasters and conflict. However, the existing nighttime light remote sensors have many limitations because they are subject to one or more shortcomings such as coarse spatial resolution, restricted swath width and lack of multi-spectral data. Therefore, we propose an optical system of imaging spectrometer based on linear variable filter. The imaging principle, optical specifications, optical design, imaging performance analysis and tolerance analysis are illustrated. The optical system with a focal length of 100 mm, F-number 4 and 43° field of view in the spectrum range of 400-1000 nm is presented, and excellent image quality is achieved. The system can obtain the multi-spectral images of eight bands with a spatial resolution of 21.5 m and a swath width of 320 km at the altitude of 500 km. Compared with the existing nighttime light remote sensors, our system possesses the advantages of high spatial and high spectral resolution, wide spectrum band and wide swath width simultaneously, greatly making up for the shortage of the present systems. The result of tolerance analysis shows our system satisfy the requirements of fabrication and alignment.

The Instrument Design of Lightweight and Large Field of View High-Resolution Hyperspectral Camera.

The design of compact hyperspectral cameras with high ground resolution and large field of view (FOV) is a challenging problem in the field of remote sensing. In this paper, the time-delayed integration (TDI) of the digital domain is applied to solve the issue of insufficient light energy brought by high spatial resolution, and a hyperspectral camera with linear variable filters suitable for digital domain TDI technology is further designed. The camera has a wavelength range of 450-950 nm, with an average spectral resolution of 10.2 nm. The paper also analyzed the effects of digital domain TDI on the signal-noise ratio (SNR) and the spectral resolution. During its working in orbits, we have obtained high-SNR images with a swath width of 150 km, and a ground sample distance (GSD) of 10 m @ 500 km. The design of the hyperspectral camera has an improved spatial resolution while reducing the cost.

Snapshot Imaging Spectrometer Based on Pixel-Level Filter Array (PFA).

Snapshot spectral imaging technology plays an important role in many fields. However, most existing snapshot imaging spectrometers have the shortcomings of a large volume or heavy computational burden. In this paper, we present a novel snapshot imaging spectrometer based on the pixel-level filter array (PFA), which can simultaneously obtain both spectral and spatial information. The system is composed of a fore-optics, a PFA, a relay lens, and a monochromatic sensor. The incoming light first forms an intermediate image on the PFA through the fore-optics. Then, the relay lens reimages the spectral images on the PFA onto the monochromatic sensor. Through the use of the PFA, we can capture a three-dimensional (spatial coordinates and wavelength) datacube in a single exposure. Compared with existing technologies, our system possesses the advantages of a simple implementation, low cost, compact structure, and high energy efficiency by removing stacked dispersive or interferometric elements. Moreover, the characteristic of the direct imaging mode ensures the low computational burden of the system, thus shortening the imaging time. The principle and design of the system are described in detail. An experimental prototype is built and field experiments are carried out to verify the feasibility of the proposed scheme.