According to requirements, a co-aperture design has been performed for the visible light remote sensing camera and the synthetic aperture radar, allowing the remote sensing satellite to acquire both visible light and radar images simultaneously. The front system is a two-mirror, no-focus system with a primary mirror diameter of 3 meters, serving to compress the beam. To avoid obstruction, the primary mirror is placed off-axis. The visible light component consists of an off-axis three mirror system, with the entrance pupil aligned with the exit pupil of the front system. All three mirrors are secondary mirrors with quadratic surfaces. The primary mirror size is 500mm, and the system's focal length is 7.22m. The overall ground resolution of the system reaches sub-meter level, with a full field of view measuring 0.8° × 0.03°. Optical design software ZEMAX was employed to evaluate the imaging quality within the visible light wavelength range. The results indicate that the spot size of the system is smaller than 13μm within each field of view. At the Nyquist frequency, the modulation transfer function (MTF) values for each field of view exceed 0.4, approaching the diffraction limit, showcasing good imaging quality. This design enhances the satellite's adaptability and observational capabilities, reduces the overall size of the instrument, and saves on manufacturing and launch costs.
Currently, most space-borne optical cameras have fixed focal length and depth of focus. In this case, the range within which the target can be clearly imaged has been pre-determined before launch. However, the distance of the target to the optical camera might be unknown or change very fast and therefore focus adjustment has to be carried out to obtain clear images. However, no matter which refocusing technique is used, focus adjustment might lag behind the object distance variation and depth of focus extension is a better way. Wave-front coding can be used to extend the depth of focus of incoherent imaging system but the surface profile of the phase mask could not be changed dynamically, which is not flexible for application. In this manuscript, by combing the variable curvature mirror (VCM) and coded imaging technique together, a new depth of focus extension technique is proposed. According to our previous studies, the focal plane could be quickly adjusted by changing the curvature radius of VCM. Compared with the curvature variation speed, the exposure time of the camera is quite long. Therefore, by adjusting the focal plane very fast in a wide range during the exposure through VCM, an equivalent coded optical transfer function having no null frequency points within bandwidth is generated and the image captured is uniformly blurred. After that, with the help of digital restoration, the clear image could be obtained. Because the focal plane could be adjusted through variable curvature mirror in the range of millimeter, the proposed method could be used to obtain clear images with greatly extended depth of focus.
By capturing a series of low-resolution images which have known or unknown sub-pixel displacement between each other, high resolution image could be reconstructed through algorithms such as IBP, POCS and so on. This technique mainly aims to solve the problem of aliasing effect caused by under-sampling but one problem exists. While applying sub-pixel shift based super-resolution reconstruction, point spread function is used to simulate the imaging process but usually the point spread function corresponding to the low-resolution imaging system is used, which does not match reconstruction in high-resolution grid. According to our previous researches, the wave-front coding technique could be used to realize single image amplification based super-resolution reconstruction because the point spread function corresponding to the high-resolution grid could be digitally generated in a more accurate way. In this manuscript, the rotationally symmetric wave-front coding technique and the sub-pixel shift based super-resolution imaging are combined together and there are two advantages. First, because of decrease of the magnitude of optical transfer function caused by wave-front coding, the aliasing effect in the intermediate images is reduced keeping pitch size unchanged. Second, while doing the reconstruction in high-resolution grid, the computed point spread function corresponding to the high-resolution grid is used, which better matches the high-resolution grid. The numerical results demonstrate that better image could be obtained by incorporating rotationally symmetric wave-front coding into sub-pixel shift based super-resolution imaging.
Phase diversity technique (PD) can jointly estimate the wavefront aberration and the target image of an optical imaging system. The PD technique reconstructs images by acquiring a focal plane image of optical system and one or more images with known aberrations (often selected defocus). Due to the simple construction of the optical system, the ability to detect discontinuous co-phase errors, and its applicability to both point sources and extended targets, The PD technique is uniquely suited for spatial target imaging applications, especially for the detection of multi-aperture piston errors. However, in a spatially low-illumination environment, Poisson noise as the main noise source of the imaging system seriously affects the accuracy of the reconstructed images. In this paper, we propose a method of phase diversity technique based on a fast Non-local Means (NLM) algorithm for reconstructing single-aperture images or multi-aperture images. For the two cases of single-aperture imaging and multi-aperture imaging with piston errors in spatial low illumination conditions, the method is used to solve the sensitivity problem of Poisson noise during image reconstruction. Numerical simulation results show that our method has significant improvement in structural similarity of the recovered images compared with the traditional phase diversity technique, and also is faster than the common non-local mean algorithm. The combination of this fast non-local means algorithm which using integral images and the phase diversity technique greatly reduce the computation time. The field experimental results and simulation results show good agreement. The new method would be useful in the AO system with active Poisson noise.
In terms of optical requirements and launch costs, large-diameter mirror should not only ensure fine surface accuracy, but also pursue high the rate of lightweight. Starting with material selection and shape design, the structure design of the 2 m mirror of a space remote sensor is carried out, and the preliminary mirror body is obtained. Then, combined with a platform of design optimization called Isight that integrated modeling software, finite element analysis software, data processing and analysis software, we optimized the key structural parameters of the mirror in detail, obtained a SiC mirror with the mass of 178 kg, its the rate of lightweight was as high as 90.9% and the RMS of surface shape accuracy under gravity deformation is 2.2 nm. On this basis, we designed and simulated the flexible support and other mirror components. The results indicated that the first-order natural frequency of the mirror components was 113.8 Hz, the RMS of surface shape accuracy was 8.1 nm under gravity deformation when the optical axis is horizontal, and 8.2 nm under the condition of 2°C temperature change, which were better than λ/60, could meet the requirement of the design index completely.
The classical Preston equation considers that the material removal is linearly related to time, velocity, and pressure. However, in the wheel polishing technology, it is found through experiments that there is a nonlinear relationship between the rotational speed of the polishing wheel and the amount of material removed. In order to accurately control the material removal in the polishing wheel variable speed machining strategy, it is necessary to modify the classical Preston equation. In this paper, the control variable method is used to carry out the sampling experiment: the time and pressure are set as fixed values, and the polishing wheel speed is set as a variable and the value is between 0-4rps. Then the data points were analyzed and a least squares fit was used to obtain a non-linear function between the rotational speed of the polishing wheel and the amount of material removed. Finally, the classical Preston equation is modified to obtain the removal equation suitable for the variable speed machining strategy.
It is difficult for normal CCD or CMOS camera to obtain high quality images under extremely low-light conditions for example the new moon or the quarter moon because the photons arriving at the detector are so few that signal to noise ratio (SNR) is much lower than what is necessary to resolve finer details in the nighttime scenario. To solve this problem, the intensified CCD or CMOS camera is adopted and the few photons is amplified to improve the SNR a lot. However, the intensifier is mainly composed of the cathode, MCP (Micro-channel-plate) and fluorescent screen and this complex structure and the multiple photoelectric conversion during the photon amplification process will lead to a big equivalent pitch size, which degrades the spatial resolution. Therefore in this manuscript, by improving the classical iterative back projection (IBP) algorithm a super-resolution reconstruction algorithm is proposed. By fusing multiple quite noisy lowlight images having sub-pixel displacements between each other, both the spatial resolution and the SNR could be enhanced. In the in-lab experiments, the spatial resolution can be increased to nearly 1.8 times the original one. Besides that, the increment in SNR bigger than 6dB and 9dB could be obtained for the quarter moon and the new moon light condition respectively. The out-door experiments show the similar results and besides that by fusing sub-pixel shifted low-light images corresponding to different low-light conditions together, the reconstructed high-resolution images will have even better visual performance.
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