At present, the space-borne optical camera installed on satellite platform used for space targets monitoring usually adopts optical system having fixed length and therefore the corresponding field of view cannot be changed either. However, searching for targets of interest needs large field of view but powerful details resolving capability depends on long focal length instead. In order to realize searching within large field of view and high-resolution imaging within small field of view by using one optical system, zooming is an ideal choice. Nowadays, optical zooming is the most popular zooming technique and by introducing zooming group and compensating group at the same time, not only the focal length could be changed but also the focal plane could be stabilized. However, mechanical moving elements based traditional optical zooming has obvious drawbacks, for example relatively low zoom speed, possible disturbance to platform stability and reliability decreasing. Therefore, optical zooming without macroscopic moving elements has been paid much attention and the key lies in the use of variable curvature mirror (VCM). By combining variable curvature mirror and optical leveraging effect, the slight variation of curvature radius of VCM can generate large optical zooming. On the one hand, the fewer the number of VCM used is, the bigger the saggitus variation of each variable curvature mirror will be. On the other hand, large zoom factor needs large saggitus variation but aberrations turn up correspondingly as well. Therefore, how to balance the number of variable curvature mirrors, the reasonable saggitus variation and better aberration compensation within limited volume are the key to system design. In this manuscript, first of all, the current development status of variable curvature mirror based optical zooming are systematically reviewed and our research progress on this technique is also introduced. After that, the design method is described and one typical design example is presented. At the same time the effectiveness of digital restoration in improving the imaging quality is demonstrated as well. Finally, the development tendency of variable curvature mirror based optical zooming is simply discussed.
In this paper, a whole general design and optimization process is detailedly demonstrated by taking the design and optimization of a 55mm diameter variable curvature mirror(VCM) with a cycloid-like thickness distribution as example. The finite-element analysis to the VCM under each change of main structure parameter is done and analyzed to choose the proper parameter value of each structure to obtain the optimum surface figure accuracy. Finally, the designed VCM can achieve 0.386mm central deflection and RMS 82.84nm within the effective aperture 28.4mm.
The camera is an important part of the optical telescope observing system, and the performance of the camera is an important factor affecting the quality and efficiency of astronomical observations. EMCCD can achieve lower noise and higher detection sensitivity by charge multiplication techniques, and can be used to realize direct observations of very faint and weak targets, and relative to the traditional CCD/CMOS detectors, the noise level can be reduced by an order of magnitude to reach the Sub-electron level. However, facing the need for calibration of ultra-low noise at the sub-electron level, it is difficult to satisfy the currently available equipment and methods. Therefore, the study of EMCCD readout noise calibration method under high gain is of great significance for the theoretical study of EMCCD and the design of low-noise electronics. In this paper, we propose a calibration system of "cascaded integrating sphere + parallel light pipe" local illumination in dark room environment, through which we can obtain the light source under ultra-low brightness, which solves the problem of difficult to obtain the point light source, and the method of local illumination can avoid the fatigue attenuation problem under the high-fold gain, and we also refine the noise model, and propose a "gain-noise" model, which can be used to calibrate the EMCCD readout noise. The noise model is refined and the "gain-noise" fitting method is proposed, and finally the readout noise test at high gain achieves a noise calibration result of about 0.8e@600x.
Multi-aperture optical systems provide a solution that can enhance resolution without the requirement for a large-diameter single-aperture system. However, one of the challenges of multi-aperture optical systems is the detection of the piston. The phase diversity (PD) technique can detect non-continuous co-phase errors and is often used for the detection of multi-aperture piston. The PD technique estimates the wavefront aberration of the optical imaging system and the target image by acquiring an image of the focal plane of an unknown target passing through the optical system and one or more images of known aberration (often chosen to be defocused). The PD technique is usually converted to a nonlinear optimization problem, but the optimization process may fall into local minima due to 2π piston ambiguity. Such a 2π piston ambiguity problem can be solved by using broadband light with multiple wavelengths. In this paper, a multi-wavelength phase diversity technique based on optimized grid search is used, which improves the detection range so that the piston and the final evaluation function values will be more likely to be within the correct range, and improves the solution success rate compared to the unoptimized grid search method.
Low-light remote sensing technology is crucial for surface observation during twilight and lunar phases; however, the acquired images often suffer from low contrast, low brightness, and low signal-to-noise ratios, which adversely affect observation quality. Traditional low-light image enhancement algorithms, such as Histogram Equalization, Gamma Correction, and Adaptive Histogram Equalization, can improve visual outcomes but also suffer from issues such as over-enhancement, loss of detail, noise amplification, and insufficient adaptability. To address these limitations, this paper proposes a low-light remote sensing image enhancement method based on Zero-Reference Deep Curve Estimation (Zero-DCE). This approach does not require paired samples and guides network learning through a non-reference loss function, making it particularly suitable for enhancing remote sensing images in low-light environments. Due to the lack of dedicated low-light remote sensing datasets, this study utilizes images from the UCMerced dataset to create simulated low-light remote sensing images for model fine-tuning. All color images are converted to grayscale to align with the characteristics of satellite-based low-light remote sensing images and to simplify the training process. Experimental results demonstrate that the proposed method significantly outperforms traditional techniques in terms of Structural Similarity Index (SSIM) and Peak Signal-to-Noise Ratio (PSNR), while also excelling in denoising and preserving texture authenticity. The optimized Zero-DCE++ not only maintains the original performance but also significantly reduces computational costs and enhances inference speed, which is of great importance for real-time low-light remote sensing image processing on satellite platforms.
With the development of space satellite remote sensing technology, the demand of high-resolution imaging is increasing day by day. The aim of this paper is to explore a new kind of optical imaging, high-resolution rotating pupil optical imaging. By introducing rectangular rotating pupil and super-resolution reconstruction algorithm into the imaging system, this technology can significantly improve the resolution of the system, reduce aberrations and improve the image quality. Firstly, the theoretical basis of rotating pupil optical imaging is analyzed, including the effect of pupil rotation on the performance of the imaging system. The degradation images with different rotation angles are obtained, and the degradation mechanism and characteristics are analyzed. Then, a set of simulation model and method for image quality improvement of rotating pupil optical imaging system are designed and constructed. The final high-resolution image is obtained by the super-resolution reconstruction algorithm, and the theoretical analysis and performance index of this technology are verified and tested. Compared with traditional active super-resolution imaging system, rotating pupil optical imaging adopts passive imaging mode, which can significantly improve imaging resolution and reduce aberrations, especially in dynamic imaging and long-distance imaging. Finally, the future development trend of rotating pupil optical imaging is prospected, and the possible improvement direction and further research suggestions are put forward. This technology is expected to play an important role in the future field of space optical imaging, providing new solutions for high-resolution imaging.
A spherical coded imaging system combined with a controlled spherical aberration lens system and a digital sharpening filter can realize a fast and low-cost extended depth of field (EDoF) imaging system. At the same time, the wavefront coding technology is introduced, which can not only extend the depth of focus of the system, but also suppress the aberration including spherical aberration in the system design. However, for the wavefront coding system, due to the modulation of the incident light wave, the light distribution is more diffuse, so the blurred image generated by the wavefront coding system is a blurred image. It is necessary to decode and restore the intermediate blurred image to obtain a clear target image. In view of the lack of convergence and reliability of IBD algorithm, the Richardson-Lucy(RL) algorithm is introduced into RL-IBD algorithm, which can effectively reduce the sensitivity of the algorithm to noise. On the basis of vector extrapolation and exponential correction, this paper proposes improvements to the RL-IBD algorithm, which enhances the stability of the algorithm, and improves the convergence speed, noise suppression ability and adaptability of the algorithm.
It is difficult for traditional CMOS camera to obtain clear images under extremely low-light conditions for example the new moon or the quarter moon because the photons generated are so few that the signal-to-noise ratio (SNR) is much lower than what is necessary to resolve finer details. Being different from traditional CMOS camera, intensified CMOS, named as ICMOS camera can greatly amplify the very limited arriving photons through external photoelectric effect and thus the corresponding SNR could be improved a lot for low-light conditions. In previous studies, by fusing a series of low-light images having sub-pixel displacement between each other through classical iterative back projection (IBP) reconstruction algorithm, not only the resolution is enhanced but also the SNR increases as well. However why the SNR can be improved through super-resolution reconstruction is not theoretically answered yet. Therefore in this manuscript two contributions are made. In the first place, the characteristics of sub-pixel super-resolution low-light imaging are firstly further investigated. By introducing the concept of spectral SNR, the analytical expression of the SNR before and after super-resolution reconstruction is established, based on which it is concluded that the MTF boosting generated by super-resolution reconstruction is one important factor that can bring in the SNR increment besides inherent noise reducing characteristic of the super-resolution reconstruction algorithm itself. In the second place, by combing the IBP based super-resolution reconstruction algorithm, the FFT (Fast Fourier Transform) based single image amplification and image enhancement methods together, better reconstruction results could be obtained.
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.
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.
Traditional imaging lidar exhibits an obvious trade-off between the resolution and the size of its optical system. In order to realize a miniaturized super-resolution (SR) imaging lidar, Fourier ptychography (FP) has been introduced to break through the diffraction limit of the camera lens. FP, derived from synthetic aperture method, is capable of acquiring high resolution and large field-of-view reconstructed images without increasing the aperture size by capturing multiple images with diverse incident angles before computationally combining with phase retrieval algorithm. In this work, a SR imaging lidar system was proposed by using reflective-type FP, which mainly consists of a s-CMOS camera, a Nd:YAG laser, and a 2-D translation stage so as to achieve aperture scanning on the x and y axes. To validate this technique experimentally, a set of images of a positive USAF chrome-on-glass target were obtained for quantitative analysis, and an uneven 1 yuan nickel-on-steel RMB coin was used to simulate the applicability of the SR imaging lidar in practical applications. The observations show that the obtained images based on FP technique have an obvious improvement in resolution, contrast, and clarity. It is worth mentioning that the resolution of these reconstructed images is increased over 3 times in the experiment on the USAF target. Moreover, the images under different apertures were collected, processed and analyzed, which suggest the initial image quality has a non-negligible influence on the reconstructed results. This technique not only improves the performance of the imaging lidar while maintaining low costs, but also bring new vitality in remote image recognition and analysis.
A variable curvature mirror (VCM) fabricated by 3D printing technique which is thickness optimized in structure design to reduce spherical aberration and supposed to be used in zoom imaging system is investigated. First, measurement and parameters fix of the mirror blank printed by 3D printing of AlSi10Mg are done for its precision deviation introduce by the manufacturing method. Second, elementary optical polishing is done for the purpose of Nickel plated. Fine optical polishing is applied on the VCM after the Nickel plated. Third, an actuation test experiment is built and tested by piezoelectric actuators of PI with nanometer precision and Zygo interferometer. The original surface figure accuracy of 90% radius is 2.225 λ / 0.394 λ (λ = 632.8 nm). As a result, within the ultimate testing range of the interferometer, the VCM achieve about 8.68μm deformation with the corresponding position change of actuator is 18μm, which is about 50% of it. Finally, an experiment of zoom imaging effect is done. The experiment shows that it does have effect to the zoom imaging which can compensate the defocus within 230.7μm. From the performance of the VCM at this stage, it can be used in infrared imaging. For the following work, its structure will be further optimized and the precision problems will be solved through using more proper manufacture method to improve its radius change performance during actuation process. Therefore, it can be used in visible light imaging in the future.
With the development of the fourier ptychography in microscopy imaging, more and more researchers consider applying fourier ptychography in long-distance imaging. However being different from microscopic imaging, long distance fourier ptychographic imaging will face more challenges, one of which is the source image quality. In this manuscript, the influence of the source image quality on camera scanning FP imaging will be investigated and simulated. In the first part, the calibration aims to solve the influence of unenenly illumination. After that, a poisson-gaussian mixed noise model based denoising is uesd and could effectively suppresses the noise through parameter estimation. Finally, Phase correalation regisitrtion has to be used to correct the mismatch between adjacent images caused by camera scanning mode. The simulation results demonstrate the effectiveness of the preprocessing methods and could make FP more robust.The signal to noise ratio could reach to 40.5dB while obtained 5×improvement in resolution.
Fourier ptychography (FP) has emerged as a powerful tool to improve spatial resolution. In order to apply FP technique to long-distance imaging for example remote sensing, many factors have to be overcome, such as diffraction, noise, turbulence and so on. In this paper, we mainly aims at studying the influence of atmospheric turbulence on FP technique, and using iterative algorithms to restore high-legible image and eliminating the residual errors, so it will meet or reach the diffraction limit of imaging system. The optical imaging systems which work in atmospheric circumstance will face the problem of imaging through atmospheric turbulence, which causing the blurring of image and badly impact the imaging capability of optical systems. We combine the FP with the theory of adaptive optics to achieve the effective recovery of the long-range target, which is subject to the effect of atmospheric turbulence. In this work, we firstly introduce a Fourier Series (FS) atmospheric phase screen generator to simulate the atmospheric-induced wave front phase distortions and represent the wave front phase as a two dimensional periodic function. Both the spatial and temporal correlations between wave-front phase screens separated by time and/or angle are properly modeled. And using the adaptive optics, we complete the correction of the atmospheric turbulence in large distance imaging through the developed algorithm. Then we propose using laser arrays coupled with coherent illumination as an effective method of improving spatial resolution in long distance images. We emulate a laser arrays realized by optical fiber conduction and also show that appropriate phase retrieval based reconstruction algorithm which can be used to effectively recover the lost high resolution details from the multiple low resolution acquired images. Finally we analyze the effects of the atmospheric turbulence on the reconstructed image quality. The results prove that under the influence of atmospheric turbulence at outer scale of 1-m, inner scale of 0.1-m, Fourier ptychographic reconstruction can obtain good image quality for object 200 meters far away. The spatial resolution is increased six-fold.
As a new kind of optical imaging technology, polarimetric imaging can be able to identify the target that may be difficult to conventional ones and can reduce the influence of stray and complex environment. It can efficiently increase the detection dimension of the information and increase capability of target imaging and recognition by imaging the polarization properties of the optical wave. The dissertation researches a type of simultaneous polarization imaging optical system with divided aperture. This system is adopted the identification system of polarization and morphological feature, which can improve the ability of space target classification and recognition. It also can be used as a space-based space target imaging system, which can be used for the classification and recognition of space target. Polarization optical system is adopted the structure mode of two-mirror reflecting systems and field correction mirror, pupil division and four zoning registration scheme of array CCD detector. The system technical parameters are F#/12.5, EFL 1500mm, FOV 0.47°. The size of CCD pixel is 12μm×12μm. The system can detect the light of 0°/45°/90° and visible light for 450-850 nm spectrum. It reached the conclusion that optical system imaging quality is close to the diffraction limit at the Nyquist frequency 41.70lp/mm though simulation test, the system can meet the imaging requirements.
An off-axis three-mirror detection system with a large field of view is designed in order to improve the space target detection capability. The optical system is a Cook-TMA with the focal length of 127mm, the F number of 2, the field angle of 25° × 25° and the spectral range of 400-700nm. The primary mirror and the tertiary mirror of the off-axis three mirror system are all designed by free form surfaces: the primary mirror is characterized by Zernike polynomial and the tertiary mirror is described by XY polynomial. At the same time, we analyze the related characteristics of Zernike polynomial and XY polynomial. The results show that the free form surfaces have great advantages in improving the field of view and the imaging quality of the off-axis optical system. The system has high energy concentration and good imaging quality, which can capture and track the target in a wide range is suitable for wide area target monitoring.
For 640 pixel×512 pixel cooled staring focal plane array detector, a VisSWIR wideband continuous zoom optical system with 7X zoom range is presented based on the pattern of the negative zoom group and compensating lens group. The zoom system provides continuous changed in the field of view from narrow to the wide. The zoom optical system works in the range of 0.4μm~1.7μm, F number is 4, the pixel of the detector is 15μm. It realizes 20mm~140mm continuous zoom with a smooth zoom path and provided high image quality with the whole zoom range, the zoom ratio is 7:1. The modulation transfer function(MTF) for the system is above 0.5 within the whole focal length range at spatial frequency of 34lp/mm and it almost approaches the diffraction limit. RMS value of spot diameter was investigation, the maximum distortion value is less than 5% and the surface type of all lens applied is spherical. Moreover, the cam curve after optimization is given by the optical design software Code V macro. The design results provide that the zoom system has the small size, high resolution, excellent image quality and the smooth cam curve etc.
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