NVST成像系统观测数据干涉条纹研究与处理
发布时间：2018-11-12 14:51
【摘要】：中国科学院云南天文台抚仙湖一米新真空太阳望远镜(New Vacuum Solar Telescope,简称NVST)是我国新一代地基太阳观测设备之一,其中Hα成像系统是进行太阳色球观测的主要终端。Hα实测数据(特别是偏带观测数据)中明显存在由薄膜干涉引起的等厚条纹,而常规平场校正有时无法将其消除,并在后期图像高分辨率统计重建过程中造成严重影响,本论文便是针对成像系统的干涉条纹展开相关的特征分析和去除工作。论文首先介绍了天文薄型CCD产生干涉条纹的基本原理(即薄膜干涉),并可视化分析了薄膜厚度、入射角度以及折射率等参数对干涉条纹的影响。同时对NVST及其多波段成像系统进行了简介,强调了望远镜各轴之间的旋转不同心和镜筒弯沉等对成像系统所造成的影响(例如入射角度的变化)。其次,集中分析了Hα偏带观测数据中干涉条纹的空间分布和时间演化特性。分析发现:干涉条纹的间距或二维空间分布样式随时间固定不变,但条纹的强度随时间变化明显(4个小时左右可见度增加9倍),这是导致常规平场校正无法将其消除的主要原因。根据该现象,结合已有的光轴变化实测结果,推断造成上述时变特性的主要原因是观测过程中入射光的强度以及入射光与CCD靶面位置的相对角度随时间发生了变化,而且产生条纹的薄膜结构与靶面位置的距离比较近(应在40mm以内),为后续从硬件改造的角度抑制干涉条纹提供参考。在上述工作的基础上,提出并尝试了两种降低条纹可见度的方法。首先对探测器(pco.4000型CCD)光敏介质的前封窗进行了改造,通过改造前封窗楔角至2度的方案来调制条纹间距至CCD像元尺度,改造之后小范围视场内可识别出可见度仅为0.6%左右的少量干涉条纹。其次基于图像滤波技术提取干涉条纹图样并生成所谓“条纹平场”,分别采用和比较了傅里叶频域滤波、小波变换滤波和空域中值滤波三种方法,结果基本相同。观测数据再次进行“条纹平场”校正之后,条纹消除效果明显:可见度可降低8倍左右(即由之前的4.7%降低为0.6%以下)。同时也指出多幅图像的积分(累加)可以有效降低精细太阳结构在“条纹平场”提取中的影响,并给出累加时间的经验值约为15-20分钟,为下一步的从高分辨统计重建算法角度抑制干涉条纹的工作提供了可借鉴的经验。最后讨论了上述滤波方法的局限性,并尝试利用Chae(2004)提出的新算法进一步减弱“条纹平场”中的太阳结构痕迹。给出了该方法在NVST上的初步实测结果,为具有强对比结构的活动区数据获取“准实时平场”来修正干涉条纹提供了新的思路。
[Abstract]:The New vacuum Solar Telescope (New Vacuum Solar Telescope,) at Fuxian Lake, Yunnan Observatory, Chinese Academy of Sciences, is one of the new generation of ground-based solar observation equipment in China. The H 伪 imaging system is the main terminal of the solar color sphere observation. The equal-thickness fringes caused by thin film interference are obviously present in the H 伪 measured data (especially the biased band observation data), which can not be eliminated by conventional flat field correction. And in the process of high resolution statistical reconstruction of image, this paper is aimed at the interference fringes of the imaging system to carry out the relevant feature analysis and removal work. In this paper, the basic principle of interference fringes produced by astronomical thin CCD (thin film interference) is introduced, and the influence of thickness, incidence angle and refractive index on the interference fringes is analyzed visually. At the same time, the NVST and its multi-band imaging system are briefly introduced, and the influence of the different rotation center between the various axes of the telescope and the deflection of the mirror tube on the imaging system (such as the change of incidence angle) is emphasized. Secondly, the spatial distribution and temporal evolution of interference fringes in H 伪 biased band data are analyzed. It is found that the spacing of interference fringes or two-dimensional spatial distribution patterns are constant with time, but the intensity of fringes varies obviously with time (the visibility increases by 9 times in about 4 hours). This is the main reason that the conventional flat field correction can not be eliminated. According to this phenomenon, combined with the measured results of optical axis variation, the main reason for the time-varying characteristic is that the intensity of incident light and the relative angle between incident light and CCD target surface have changed with time. Moreover, the distance between the film structure and the target position is close (within 40mm), which provides a reference for further hardware transformation to suppress interference fringes. Based on the above work, two methods to reduce the visibility of stripes are proposed and tried. In this paper, the front sealing window of Guang Min medium of the detector (pco.4000 type CCD) is first modified. The fringe spacing is modulated to the size of CCD pixel by modifying the front sealing window with wedge angle to 2 degrees. A small number of interference fringes with a visibility of only 0.6% can be identified in the small field of view after the transformation. Secondly, the interference fringe pattern is extracted based on the image filtering technique and the so-called "fringe flat field" is generated. Three methods, Fourier frequency domain filtering, wavelet transform filtering and spatial median filtering, are adopted and compared, respectively, and the results are basically the same. After the "fringe flat field" correction is carried out again, the fringe elimination effect is obvious: the visibility can be reduced about 8 times (that is, from 4.7% to less than 0.6%). At the same time, it is pointed out that the integration (accumulation) of multiple images can effectively reduce the influence of fine solar structure on the "fringe flat field" extraction, and the empirical value of the accumulated time is about 15-20 minutes. It provides the experience for the next step to suppress interference fringes from the angle of high resolution statistical reconstruction algorithm. Finally, the limitations of the above filtering methods are discussed, and the new algorithm proposed by Chae (2004) is used to further weaken the trace of the solar structure in the "fringe flat field". The preliminary experimental results of the method on NVST are given, which provides a new idea for obtaining the "quasi-real-time flat field" from the active region data with strong contrast structure to correct the interference fringes.
【学位授予单位】：中国科学院研究生院(云南天文台)
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：P111.41
[Abstract]:The New vacuum Solar Telescope (New Vacuum Solar Telescope,) at Fuxian Lake, Yunnan Observatory, Chinese Academy of Sciences, is one of the new generation of ground-based solar observation equipment in China. The H 伪 imaging system is the main terminal of the solar color sphere observation. The equal-thickness fringes caused by thin film interference are obviously present in the H 伪 measured data (especially the biased band observation data), which can not be eliminated by conventional flat field correction. And in the process of high resolution statistical reconstruction of image, this paper is aimed at the interference fringes of the imaging system to carry out the relevant feature analysis and removal work. In this paper, the basic principle of interference fringes produced by astronomical thin CCD (thin film interference) is introduced, and the influence of thickness, incidence angle and refractive index on the interference fringes is analyzed visually. At the same time, the NVST and its multi-band imaging system are briefly introduced, and the influence of the different rotation center between the various axes of the telescope and the deflection of the mirror tube on the imaging system (such as the change of incidence angle) is emphasized. Secondly, the spatial distribution and temporal evolution of interference fringes in H 伪 biased band data are analyzed. It is found that the spacing of interference fringes or two-dimensional spatial distribution patterns are constant with time, but the intensity of fringes varies obviously with time (the visibility increases by 9 times in about 4 hours). This is the main reason that the conventional flat field correction can not be eliminated. According to this phenomenon, combined with the measured results of optical axis variation, the main reason for the time-varying characteristic is that the intensity of incident light and the relative angle between incident light and CCD target surface have changed with time. Moreover, the distance between the film structure and the target position is close (within 40mm), which provides a reference for further hardware transformation to suppress interference fringes. Based on the above work, two methods to reduce the visibility of stripes are proposed and tried. In this paper, the front sealing window of Guang Min medium of the detector (pco.4000 type CCD) is first modified. The fringe spacing is modulated to the size of CCD pixel by modifying the front sealing window with wedge angle to 2 degrees. A small number of interference fringes with a visibility of only 0.6% can be identified in the small field of view after the transformation. Secondly, the interference fringe pattern is extracted based on the image filtering technique and the so-called "fringe flat field" is generated. Three methods, Fourier frequency domain filtering, wavelet transform filtering and spatial median filtering, are adopted and compared, respectively, and the results are basically the same. After the "fringe flat field" correction is carried out again, the fringe elimination effect is obvious: the visibility can be reduced about 8 times (that is, from 4.7% to less than 0.6%). At the same time, it is pointed out that the integration (accumulation) of multiple images can effectively reduce the influence of fine solar structure on the "fringe flat field" extraction, and the empirical value of the accumulated time is about 15-20 minutes. It provides the experience for the next step to suppress interference fringes from the angle of high resolution statistical reconstruction algorithm. Finally, the limitations of the above filtering methods are discussed, and the new algorithm proposed by Chae (2004) is used to further weaken the trace of the solar structure in the "fringe flat field". The preliminary experimental results of the method on NVST are given, which provides a new idea for obtaining the "quasi-real-time flat field" from the active region data with strong contrast structure to correct the interference fringes.
【学位授予单位】：中国科学院研究生院(云南天文台)
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：P111.41
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