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.
Synthetic aperture is the mainstream structure of current astronomical telescopes. However, after the synthetic aperture telescope is deployed in orbit, there will remain tilt and piston error between adjacent segments, which will sharply deteriorate the imaging quality of the optical system. The traditional piston error detection method based on dispersed fringe sensor has the question that it is difficult to detect the piston error within one wavelength, and the detection accuracy is restricted by the detection range. The method in this paper constructs multiple monochromatic light channels by opening windows in different areas on the dispersed fringe pattern, calculating and obtaining the feature value in each window to form a feature vector. Then, the convolutional neural network is introduced to distinguish the feature vector to detect piston error. Among them, the training set construction method adopted in this paper only needs raw data in one wavelength to construct a training set covering the entire detection range. Through simulation, the method proposed in this paper achieves the detection range of [-208λ, 208λ] (λ=720nm), and regardless of the presence of noise, the root mean square value of the detection error does not exceed 17.7nm (0.027λmin, λmin=660nm).
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.
Many factors could lead to deviation of focal plane of a space-borne camera from its ideal position and thus on-orbit focusing are indispensable to capture satisfactory images of space targets. Among all typical focusing techniques, changing the position of focal plane directly through motor-driven worm and gear is the simplest one, but two drawbacks are obvious. First, mechanical movement is slow but the space targets usually move very fast. In this case, it is highly probable that focus adjusting is always lagging. Second, the targets especially the non-cooperative ones may appear anywhere and working distance of defocus compensation should be large enough which makes the focusing assembly much heavier. Factually, most large aperture space-borne cameras are all-reflective or all-reflective having lens correctors, therefore by changing the interval between the primary and secondary mirror or by changing intervals within lens correctors defocus could be compensated. Although the sensitiveness is improved, moving elements are still needed indicating underlying lagging. Therefore in this manuscript, a new focus adjusting method is proposed. By changing the secondary mirror into a variable curvature mirror (VCM), the defocus compensation could be realized by varying the curvature radius of VCM. One prototype space-borne optical camera whose focal length is 6000mm and aperture is 600mm is used to verify the method. Our research demonstrates that the VCM based focus adjusting is not only very sensitive but also suitable for very severe defocus. Specifically, only a slight saggitus variation of less than 4um could compensate amazing defocus of about 4mm while maintaining good linearity between the saggitus variation of VCM and defocus, which proves the potential of this focus adjusting method.
Phase diversity technique (PD) is a widely known method to estimate wave-front aberration of optical imaging system and to obtain reconstructed high-resolution image after degradation. However, when detecting weak or low light object in space, Poisson noise, as the main source of noise, has a serious impact on the accuracy of the PD’s two main function. Hence, we firstly propose a modified PD combined with Non-local Means (NLM) algorithm to reduce the sensitivity of PD towards the Poisson noise. The numerical simulations demonstrate that our approach compared with the traditional PD has a significant improvement in terms of both the wave-front residual root-mean-square error (RMS) and the structural similarity index metrics (SSIM). The wave-front residual RMS decreases by approximately 51% across the Poisson noise levels ranging from 24.48 dB to 61.02 dB. The overall decline range of SSIM significantly decreases from 47% to 17%, and the average of SSIM increases from 84% to 91%. The modified PD would be useful in the AO system with active Poisson noise.
Variable curvature mirror (VCM) is a long-history technique used to correct the defocus and spherical aberrations caused by thermal lens effect in solid-state laser. In recent years, the probability of VCM in realizing non-moving element optical zoom imaging has been paid much attention and how to generate a large enough saggitus variation while still maintaining good enough surface figure accuracy is the research hot topic. In this manuscript, two kinds of VCM has been studied and the advantages of pressurization actuation based VCM having variable mirror thickness has been confirmed. Compared with the traditional annular force actuation based VCM with constant mirror thickness, the pressurization actuation based one having variable mirror thickness is capable of providing a saggitus variation of larger than 35um and still maintaining its surface figure accuracy superior to 1/10λ(λ=632.8nm). Besides that, it is found that spherical aberration plays a main role in leading to the degradation of surface figure accuracy and the surface figure accuracy at extreme curvature could be improved to about 1/40λ(λ=632.8nm) by only removing spherical aberration. Therefore, when applying pressurization actuation based VCM to realize non-moving element optical zooming, the wavefront sensing and subsequent digital correction to eliminate the spherical aberration will become a necessary step.
Variable curvature mirror (VCM) can change its curvature radius dynamically and is usually used to correct the defocus and spherical aberration caused by thermal lens effect to improve the output beam quality of high power solid-state laser. Recently, the probable application of VCM in realizing non-moving element optical zoom imaging in visible band has been paid much attention. The basic requirement for VCM lies in that it should provide a large enough saggitus variation and still maintains a high enough surface figure at the same time. Therefore in this manuscript, by combing the pressurization based actuation with a variable thickness mirror design, the purpose of obtaining large saggitus variation and maintaining quite good surface figure accuracy at the same time could be achieved. A prototype zoom mirror with diameter of 120mm and central thickness of 8mm is designed, fabricated and tested. Experimental results demonstrate that the zoom mirror having an initial surface figure accuracy superior to 1/80λ could provide bigger than 36um saggitus variation and after finishing the curvature variation its surface figure accuracy could still be superior to 1/40λ with the spherical aberration removed, which proves that the effectiveness of the theoretical design.
Super-resolution image reconstruction is a process to reconstruct high-resolution images from shifted, low-resolution, degraded observations. In the last two decades, a variety of super-resolution methods have been proposed. These methods are usually very sensitive to their assumed model of data and noise, which limits their utility. This paper reviews some of these methods and addresses their shortcomings. We propose an alternate approach using 1norm minimization and robust regularization based on a bilateral prior to deal with different data and noise models. This computationally inexpensive method is robust to errors in motion and blur estimation and results in images with sharp edges. Experimental results confirm the effectiveness of our method and demonstrate its superiority to other super-resolution methods.
In recent years, a novel optical zooming technique has been paid much attention. With the help of optical leveraging effect, it is possible to alter the system focal length dramatically without moving elements involved in by only changing the curvature radius of VCM (variable curvature mirror) slightly. With no doubt, VCM is the key to realize non-moving element optical zooming and it has to provide large enough saggitus variation while still maintaining the high surface figure accuracy to ensure high quality imaging. In our previously published paper, an annular force based VCM has been designed, fabricated and tested. Experiments demonstrate that with the aperture of 100mm and thickness of 2mm, the VCM could generate a large saggitus variation exceeding 30λ (λ=632.8nm). However, the optical quality degrades very fast and this makes such a VCM unsuitable for optical imaging in visible band. Therefore in this manuscript, a multipoint actuation array, which is composed of totally 49 piezoelectric actuators, is embedded into the annular structure to aim to correct the surface figure distortion caused by large saggitus variation. The new structure model has been designed and numerical simulation indicates that the surface figure distortion could be well corrected as long as the degraded surface figure accuracy is better than 1.8λ (λ=632.8nm) (RMS). Based on this, a new prototype VCM is being fabricated and intermediate results are reported here.
KEYWORDS: Super resolution, Imaging systems, Point spread functions, Image quality, Sensors, Image resolution, Modulation transfer functions, Image restoration, Phase transfer function, Optical transfer functions
Wave-front coding, proposed by Dowski and Cathey in 1995, is widely known to be capable of extending the depth of focus (DOF) of incoherent imaging systems. However, benefiting from its very large point spread function (PSF) generated by a suitably designed phase mask that is added to the aperture plane, wave-front coding could also be used to achieve super-resolution without replacing the current sensor with one of smaller pitch size. An image amplification based super-resolution reconstruction procedure has been specifically designed for wave-front coded imaging systems and its effectiveness has been tested by experiment. For instance, for a focal length of 50 mm and f-number 4.5, objects within the range [5 m, ∞] are clearly imaged with the help of wave-front coding, which indicates a DOF extension ratio of approximately 20. The proposed super-resolution reconstruction procedure produces at least 3× resolution improvement, with the quality of the reconstructed super-resolution image approaching the diffraction limit.
Recently, a new kind of optical zooming technique in which no moving elements are involved has been paid much attention. The elimination of moving elements makes optical zooming suitable for applications which has exacting requirements in space, power cost and system stability. The mobile phone and the space-borne camera are two typical examples. The key to realize non-moving elements optical zooming lies in the introduction of variable curvature mirror (VCM) whose radius of curvature could be changed dynamically. When VCM is about to be used to implement optical zoom imaging, two characteristics should be ensured. First, VCM has to provide large enough saggitus variation in order to obtain a big magnification ratio. Second, after the radius of curvature has been changed, the corresponding surface figure accuracy should still be maintained superior to a threshold level to make the high quality imaging possible. In this manuscript, based on the elasticity theory, the physical model of the annular force based variable curvature mirror is established and numerically analyzed. The results demonstrate that when the annular force is applied at the half-the-aperture position, the actuation force is reduced and a smaller actuation force is required to generate the saggitus variation and thus the maintenance of surface figure accuracy becomes easier during the variation of radius of curvature. Besides that, a prototype VCM, whose diameter and thickness are 100mm and 3mm respectively, have been fabricated and the maximum saggitus variation that could be obtained approaches more than 30 wavelengths. At the same time, the degradation of surface figure accuracy is weakly correlated to the curvature radius variation. Keywords: optical zooming; variable curvature mirror; surface figure accuracy; saggitus;
The key to realize non-moving-element optical zooming lies in VCM (variable curvature mirror). In order to obtain a large optical magnification, VCM should be capable of providing a large center deflection and this requires that the mirror material should be robust enough, require less force to deform and have a high ultimate strength. In this paper, CFRP (carbon-fiber-reinforced-polymer) is selected as the mirror material and a prototype VCM has been fabricated. With diameter of 100mm, thickness of 2mm and initial curvature radius of 1740mm, this VCM can provide a center deflection approaching nearly 23um, which proves the feasibility of CFRP in constructing VCM. Compared with the work reported in [Proc. of SPIE, 8725, 87250W, 2013], the center deflection obtained here becomes even larger.
Variable curvature mirror (VCM) is a simplified active optical component being capable of changing its curvature radius.
Curvature radius variation within a wide range requires that the VCM should be able to generate a large saggitus
variation. Besides that, the surface form accuracy should be maintained above a reasonable level. In this paper, a
piezoelectric actuation based prototype VCM is designed, constructed and experimentally tested. The thickness of the K9
plane mirror is only 3mm over the full aperture of 100mm. Six piezoelectric actuators are fixed into a base plate and the
head of each actuator is connected to an annular ring through the screw thread. With such a structure, the force provided
by each actuator can be transformed to the mirror backside through this annular ring. With each actuator generating the
same force, the curvature radius can be changed in a uniform way. At the mean time, the surface form accuracy could be
adjusted one point by point to compensation asymmetric modes as well. Mathematical analysis and FEA (finite element
analysis) are used together to demonstrate the theoretical correctness. Besides that, the prototype VCM is successfully
constructed and experiments have been carried out to give a quantitative assessment on the saggitus variation.
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