When the space optical remote sensing system works in orbit, it is easy to be affected by the external environment such as vibration and shock, space environment and optical system itself. In this case, the position of the secondary mirror relative to the primary mirror will shift, which causes wavefront aberrations and deteriorates the image quality. The traditional position errors detection technique has the following disadvantages such as complex device, time-consuming calculation, low accuracy and small dynamic range. In view of the shortcomings of the existing secondary mirror position errors detection technology, a large dynamic range secondary mirror position errors detection method based on two-branch convolutional neural network (CNN) is proposed. Taking the coaxial three-mirror system as the research object, its maximum RMS value of offset wavefront aberration is above 6 λ. This method uses a new dual-branch CNN to establish the nonlinear relationship between symmetrical defocus point spread function (PSF) and the secondary mirror position offset under a single field of view, which improves the dynamic range of secondary mirror position errors detection. The prediction effects of the network trained with different fields of view data are compared, and the effects of field deviation and environmental noise on the prediction accuracy of the network are analyzed. The simulation results show that the proposed method can effectively improve the dynamic range of secondary mirror position errors detection, also has high sensing accuracy, and has good generalization ability.
As a widely used wavefront sensor in adaptive optics, Hartmann-Shack wavefront sensor (HSWFS) has a simple structure and high accuracy. However, the dynamic range and field of view of HSWFS are limited, preventing it from usage under strong turbulence conditions. Plenoptic wavefront sensor (PWFS) has the advantages of large dynamic range and detection field of view, which effectively compensates for the shortcomings of HSWFS. In this paper, we analyzed the performance of the PWFS theoretically and then verified by simulation. Because of the fundamental error in wavefront slope measurement of PWFS, the open-loop wavefront correction accuracy is unsatisfied and closed-loop correction is highly preferred.
Space-based high-resolution imaging system has a trend to broadband spectrum imaging in geosynchronous orbits. The resolution of an imaging system is related to the aperture size, the height of the orbit of the system, and the detection wavelength. For traditional imaging system, it is difficult to increase the aperture size due to the limitation of manufacturing level, processing cost and load weight. The optical sparse aperture (OSA) system uses a number of independent subaperture systems with a smaller size and the same standard to collect light from objects and synthetizes a blur image in focal plane, which can achieve a large equivalent aperture size. However, with the increasing number of the sub-aperture systems, the system structure becomes more complicated, which is difficult to be used in practice. This paper presents an approach to search for an optimal OSA system based on annular pupil structure and reduce the number of sub-aperture systems by rotating the pupil to compensate PSF information and improve the image quality in OSA system. In this paper, a 12-aperture OSA system applied for earth observation in geosynchronous orbit are designed optimally to realize the resolution of 1m. The optimal scheme for rotating the pupil with twelve apertures under the threesymmetric structure was determined, and the MTF of the hexagonal structure was obtained. The simulation results show that the method can be applied to design a OSA system with a rotating pupil, synthesize all images within the rotation period, collect all frequency information, and restore high-resolution images using image restoration techniques.
The precise alignment of the space telescope with an active secondary mirror (ASM) is a key factor to ensure the imaging quality. The traditional alignment methods, like sensitivity table method and SPGD, are not suitable for on-orbit instant alignment of space telescope. In this paper, a model-based wavefront sensorless adaptive optics (WSLAO) method is proposed for the alignment of ASM of space telescope. This method does not require an additional wavefront sensor (like Shack-Hartmann) or a lot of iterations (like phase retrieval). The low spatial frequency content of images is used as the metric function. The misalignment errors can be estimated from the quadratic relationship between the metric function and the coefficients of gradient orthogonal modes (GOM). Here GOM is derived from the response functions of five degrees of freedom of the ASM. A three-mirror anastigmatic telescope with an ASM is modeled for the performance tests. Simulation results show high corrective accuracy and rapid convergence of our method. The RMS of the residual aberration can be reduced to less than 0.02 rad after three correction cycles.
High-resolution space observation is of great importance for scientific and military use. To get higher resolution, a larger imaging aperture is highly required. For example, to obtain one-meter ground sampling distance (GSD) in visible band on geostationary orbit (GEO), the pupil diameter of space telescope is around 25 meters. Trying to fabricate and launch so large monolithic mirror will meet many unconquerable obstacles. A feasible scheme is using sparse aperture imaging technique based on small satellites formation. This paper is focused on a sparse aperture telescope consisting of small sub-telescopes to form a Fizeau imaging interferometer. Each sub-telecope is based on a small satellite. Imaging performance of an annular structure consisting of 25 sub-apertures is evaluated by simulation. The influence of phasing error (including piston and tip/tilt) of subapertures on image quality is evaluated. The co-phasing error budget of sparse aperture telescope dependent on field of view is also analyzed. A co-phasing error detection and correction method based on wavefront sensorless adaptive optics (WSLAO) is proposed at last.
The traditional Shack-Hartmann wavefront sensor requires that the focal spot of each microlens must remain in its corresponding sub-aperture range to avoid mistakes in spot-subaperture matching. We present a software-based recognition algorithm that can obtain a much larger dynamic range while maintaining high precision: Iterative extrapolation method. In order to find the corresponding spots of all the subapertures, the method first select a 3x3 spot-array and establishes a polynomial function about the spot position to predict and find the adjacent spots, and then carry on this procedure in successive steps of the iterative algorithm. The performance of the iterative extrapolation method to expand the dynamic range of various wavefront are studied and compared with the sorting method by simulation. Finally, experiments were carried out to further verify the performance of the method. Both simulation and experimental results show that this algorithm can effectively expand the dynamic range of SHWS and the deviation of reconstructed wavefront from ideal one is below 0.08λ(PV).
A hybrid phase retrieval (HPR) method using a combination of linear phase retrieval (LPR) and iterative phase retrieval (IPR) is proposed for real-time wavefront sensing. Only the intensity information of a single defocused image is required by HPR. Low-order aberrations are estimated by classical LPR algorithm but using a “segmented” detector to provide design flexibility and better sensing accuracy. High-order aberrations are estimated by a kind of modified Gerchberg-Saxton (MGS) algorithm which uses the LPR result as a prior knowledge to significantly speed the convergence. The performance of HPR is tested under various seeing conditions by simulation. For atmospheric aberrations with D/r0=3, HPR containing LPR and ten-iteration IPR can achieve an averaged Strehl ratio of 0.88.
The Modified Hybrid-Input-Output (MHIO) phase retrieval algorithm is proposed for wavefront sensing. The results show that the MHIO algorithm significantly outperforms the Modified Gerchberg-Saxton algorithm (MGS) in large noise. However its dynamic-range is lower than MGS algorithm. It also shows that if combine the MGS algorithm with MHIO algorithm, which is called MGS+MHIO algorithm, then it can retain the property of MGS’s high dynamic-range and MHIO’s accuracy so that outperforms either MGS or MHIO algorithm. Repeating simulation results show that MGS+MHIO algorithm improves RMS of phase error obviously in high dynamic range and large noise.
Large-aperture segmented primary mirror will be widely used in next-generation space-based and ground-based telescopes. The effects of intersegment gaps, obstructions, position and figure errors of segments, which are all involved in the pupil plane, on the image quality metric should be analyzed using diffractive imaging theory. Traditional Fast Fourier Transform (FFT) method is very time-consuming and costs a lot of memory especially in dealing with large pupil-sampling matrix. A Partial Fourier Transform (PFT) method is first proposed to substantially speed up the computation and reduce memory usage for diffractive imaging analysis. Diffraction effects of a 6-meter segmented mirror including 18 hexagonal segments are simulated and analyzed using PFT method. The influence of intersegment gaps and position errors of segments on Strehl ratio is quantitatively analyzed by computing the Point Spread Function (PSF). By comparing simulation results with theoretical results, the correctness and feasibility of PFT method is confirmed.
In the process of high-resolution astronomical observation and space optical mapping, the wavefront aberrations caused by atmosphere turbulence effect lead to reduced resolution of optical imaging sensor. Firstly, on the base of influence of atmosphere turbulence effect for the optical observation system, this paper investigates and analyses the development and technical characteristics of deformable mirror, which is the key device of optical wavefront control technology. In this part, the paper describes the basic principles of wavefront control and measurement using the current production line of deformable mirror, including micro-electromechanical systems (MEMS) deformable mirror which is one of the most promising technology for wavefront modulation and Shack-Hartmann wavefront sensors. Secondly, a new method based on the technology of optical wavefront control and the data of optical path difference (OPD) for simulating the effect of optical transmission induced by turbulence is presented in this paper. The modeling and characteristics of atmosphere turbulence effect applied for optical imagery detector of astronomical observation and space optical mapping has been obtained. Finally, based on the theory model of atmosphere turbulence effects and digital simulation results, a preliminary experiment was done and the results verify the feasibility of the new method. The OPD data corresponding to optical propagation effect through turbulent atmosphere can be achieved by the calculation based on the method of ray-tracing and principle of physical optics. It is a common practice to decompose aberrated wavefronts in series over the Zernike polynomials. These data will be applied to the drive and control of the deformable mirror. This kind of simulation method can be applied to simulate the optical distortions effect, such as the dithering and excursion of light spot, in the space based earth observation with the influence of turbulent atmosphere. With the help of the optical wavefront control technology, the optical sensor and ability of space optical detection system for correcting the target image blurred by turbulence of atmosphere can be tested and evaluated in the laboratory.
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