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题名:
扩展目标高分辩率斑点成像的研究
作者: 杨连臣
学位类别: 博士
答辩日期: 2001
授予单位: 中国科学院光电技术研究所
授予地点: 中国科学院光电技术研究所
导师: 沈忙作
关键词: 大气湍流 ; 斑点成像 ; 计算机模拟 ; 小波变换 ; 图像重建
中文摘要: 斑点成像技术(前称斑点干涉术)是克服大气湍流影响,提高地面大口径望远镜分辨本领的为数不多的有效途径之一。这项技术自产生以来,首先在天文观测上得到了广泛应用。近些年来,这一方法在军事上的应用得到了关注,因为它能提供人造地球卫星的高分辨率图像,对于空间目标的侦察与识别有着重要的意义。为了在我国开展对人造卫星的高分辨率斑点成像技术的应用研究,也为今后高分辨率望远镜中斑点成像系统的设计和研制提供必要的依据,必须对斑点成像技术进行深入的研究。斑点成像系统是一个相当复杂的系统,数学解析的方法只能对某个环节或某些参数的影响进行描述,对整个系统多方面因素的综合效果却缺乏直观、有效的手段。本文采用比较可行的计算机数值模拟方法,研究斑点成像的整个过程,探讨实现扩展目标高分辨率成像的条件与方法。 本文首先模拟了能代表一定统计特性的大气湍流效应,以及包括望远镜口径、观测波长和像面探测器的成像系统。用次谐波方法成功地对大气湍流的低频部分进行了补偿。用生成的相位屏模拟了点目标的短曝光像,也即系统的点扩散函数,并计算了大气-望远镜复合传递函数和长、短曝光的光学传递函数。以双星和和扩展目标(卫星)两个目标为例,分别用交叉谱(Cross Spectrum)和双谱方法(Bispectrum)对斑点成像的整个过程进行了研究。在恢复目标振幅谱和相位谱以及重构目标的过程中,研究了高频部分取值较小,甚至为零值时,所带来的旁瓣噪声的影响及克服方法。相位恢复中,对相位缠绕问题、迭代步长的选择以及算法的优化等进行了研究。在双谱恢复方法中,为了减少计算资源消耗,结合了计算机层析技术。模拟了有限光子数的斑点成像过程。分析了不同光子水平图像频谱的信噪比,计算了有限光子数时望远镜与大气复合系统的传递函数,以及照片幅数,光子水平与系统传递函数截止频率的关系,同时还研究了恢复图像点扩展函数的信噪比问题。从另一个角度推导了有限光子数情况下的交叉谱无偏估计表达式,研究了有偏项对振幅谱和相位谱的影响。并给出了卫星和双星的恢复情况。模拟实验研究表明,影响恢复图像效果的首要的和关键的因素是光子水平。记录和处理短曝光图像的数目是恢复扩展目标分辨率的另一个因素,但是能够明显改善恢复质量的短曝光像的数目存在一个限度。对含有CCD噪声的有限光子数短曝光像的恢复进行了研究。提出了一种改进的振幅谱恢复方法,推导了有限光子数和额外CCD噪声存在情况下的交叉谱无偏估计表达式,分析了CCD噪声对振幅谱和相位谱恢复的影响,并且研究了不同CCD噪声情况下的点扩散函数恢复信噪比。依据CCD参数的变化,研究了CCD噪声对图像恢复质量的影响。最后给出了双星和卫星目标的恢复情况。结果发现,短曝光图像每个像素的信号电子数与CCD噪声电子数之比(像素信噪比)对恢复质量起关键作用。分析了不同光子水平和CCD噪声存在情况下的扩展目标斑点成像的分辨探测能力。得出了恢复接近衍射极限像的条件表达式,并用卫星的恢复结果进行了验证。分析和模拟结果表明,目标可分辨单元数、光子水平和CCD噪声强度是扩展目标恢复的几个关键因素。提出了利用小波变换对斑点成像进行多分辨率分析的方法。结果显示本方法能够揭示目标不同尺度方向上的特征,而且只利用低频成分就能恢复出满意的目标。这一结果揭示了这种
英文摘要: Speckle imaging technique (formerly named Speckle interferometry) is one of the several effective approaches, which can overcome atmosphere turbulence effects to improve resolution power of the ground-based telescope. After the technique came into being, it was first used in astronomical observations widely. Particularly in recent years, its applications in military fields attract abroad attentions because it can provide high-resolution satellite images. All of this is very important for space object reconnaissance and recognition. In order to conduct satellite high-resolution speckle imaging application research, and to provide necessary basis for developing of speckle imaging system, we need to make an in-depth study on the speckle imaging technique. Speckle imaging system is a very complicated system. Mathematical analytic method can only describe the influences of some system parameters; it lacks intuitionistic, effective means for synthetically analyzing the entire system. In this paper, the feasible computer simulation method was used to investigate the whole speckle imaging process, to probe into the realization and of conditions for high-resolution imaging of extended objects. Random phase screen with atmosphere turbulence statistics property, telescope aperture size, observation wavelength and image detector were simulated. In the simulation, subharmonic method was used to compensate low frequency part of the turbulence. Point short exposure image was simulated using phase screen, system's point spread function (PSF), and atmosphere-telescope transfer function were computed, which are close to the theoretical prediction respectively. The cross spectrum and bispectrum speckle imaging were studied on the basis of two object models: binary star and satellite. During the entire restoration of amplitude spectrum, phase spectrum and object itself, the side lobe noise problem resulting from the very small, or even zero, value of the transfer function at high frequency portion was resolved. In the phase restoration, phase-wrapped problem, recursive step choice and algorithm optimization were investigated. When using the bispectrum method to restore the phase, computer tomography (CT) technique was combined in order to lessen the computing time. The photon limited speckle imaging was also simulated. Various photon level short exposure image signal-noise-ratio (SNR), atmosphere-telescope system transfer function were computed. The relations between frame numbers of short exposure images, photon levels and system transfer function cutoff frequency was given. The unbiased estimation expression of photon limited short exposure image cross spectrum was deduced from another point of view. The binary stars and satellite restorations images at different photon level were presented. The simulation results show that the dominant factor is photon level, and the frame number of short exposure images to be processed is another one for the restoration quality. But there is a frame numbers limit, beyond which the image quality can be hardly improved. The effects of CCD detector noise on image restoration at low photon level were studied. A modified amplitude spectrum method was brought forward, and unbiased estimation cross spectrum expression under limited photon level and CCD noise was obtained. The CCD noise effects on amplitude and phase spectrum restoration were investigated. The SNR of system's PSF under various CCD noise and photon levels was also studied. At last the binary star and satellite image restoration according different photon level and CCD noise variance were showed. The results show that the ratio of effective signal electron number and root mean square electron number of CCD (CCD noise) per pixel is the key factor for the restoration quality. Resolution power of extended object speckle imaging technique was investigated based on different photon level and CCD noise intensity. The condition for the detection of diffraction-limited details was obtained, and the satellite simulations validated it. The results show that photon level, CCD noise intensity and object resolved element are the key factors for the extended object restoration. A method using wavelet transform multiresolution for speckle imaging was proposed. The results showed that the method could reveal the different scale-direction characteristics. And low-frequency band could bring almost equally satisfied object image. This also implied that the method could be used to compress the short exposure images data and reduce the compute complexity.
语种: 中文
内容类型: 学位论文
URI标识: http://ir.ioe.ac.cn/handle/181551/59
Appears in Collections:光电技术研究所博硕士论文_学位论文

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Recommended Citation:
杨连臣. 扩展目标高分辩率斑点成像的研究[D]. 中国科学院光电技术研究所. 中国科学院光电技术研究所. 2001.
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