低频射电望远镜阵列宽视场成像技术研究
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摘要
低频射电望远镜阵列宽视场成像正面临着一系列难点问题,其中最关键的问题是非共面基线效应.它的存在使得忽略w项将导致最终图像出现畸变,且随着视场的增大而加重.综述并剖析了几种w项改正算法及其技术原理,并分析了它们的计算成本和计算复杂度,进而分析比较了它们的优缺点.以平方公里阵(Square Kilometre Array,SKA)射电望远镜第1阶段低频阵列为研究对象,选取faceting和w-projection成像算法进行了仿真实验.与传统的二维傅立叶变换成像算法进行对比,分析了它们的成像质量和正确性,结果表明这两种算法在宽视场成像方面均明显优于二维傅立叶变换方法.还具体分析了分面(facet)的数目对faceting成像质量和运行时间的影响,以及w步数对w-projection成像质量和运行时间的影响,表明facet数目和w步数的选择必须合理.最后,分析了数据量大小对这两种成像算法运行时间的影响,表明这两种算法在进行海量数据处理前,需要作算法优化改进.研究结果为后续进一步综合分析宽视场成像技术以及这些技术的实用性研究提供了有价值的参考.
Wide-field imaging of low-frequency radio telescopes are subject to a number of difficult problems. One particularly pernicious problem is the non-coplanar baseline effect. It will lead to distortion of the final image when the phase of w direction called w-term is ignored. The image degradation effects are amplified for telescopes with the wide field of view. This paper summarizes and analyzes several w-term correction methods and their technical principles. Their advantages and disadvantages have been analyzed after comparing their computational cost and computational complexity. We conduct simulations with two of these methods, faceting and w-projection,based on the configuration of the first-phase Square Kilometre Array(SKA) low frequency array. The resulted images are also compared with the two-dimensional Fourier transform method. The results show that image quality and correctness derived from both faceting and w-projection are better than the two-dimensional Fourier transform method in wide-field imaging. The image quality and run time affected by the number of facets and w steps have been evaluated. The results indicate that the number of facets and w steps must be reasonable. Finally, we analyze the effect of data size on the run time of faceting and w-projection. The results show that faceting and w-projection need to be optimized before the massive amounts of data processing. The research of the present paper initiates the analysis of wide-field imaging techniques and their application in the existing and future low-frequency array, and fosters the application and promotion to much broader fields.
Wide-field imaging of low-frequency radio telescopes are subject to a number of difficult problems. One particularly pernicious problem is the non-coplanar baseline effect. It will lead to distortion of the final image when the phase of w direction called w-term is ignored. The image degradation effects are amplified for telescopes with the wide field of view. This paper summarizes and analyzes several w-term correction methods and their technical principles. Their advantages and disadvantages have been analyzed after comparing their computational cost and computational complexity. We conduct simulations with two of these methods, faceting and w-projection,based on the configuration of the first-phase Square Kilometre Array(SKA) low frequency array. The resulted images are also compared with the two-dimensional Fourier transform method. The results show that image quality and correctness derived from both faceting and w-projection are better than the two-dimensional Fourier transform method in wide-field imaging. The image quality and run time affected by the number of facets and w steps have been evaluated. The results indicate that the number of facets and w steps must be reasonable. Finally, we analyze the effect of data size on the run time of faceting and w-projection. The results show that faceting and w-projection need to be optimized before the massive amounts of data processing. The research of the present paper initiates the analysis of wide-field imaging techniques and their application in the existing and future low-frequency array, and fosters the application and promotion to much broader fields.
引文
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[2]Lonsdale C J,Cappallo R J,Morales M F,et al.Proceedings of the IEEE,2009,97:1497
[3]Tingay S J,Goeke R,Bowman J D,et al.PASA,2013,30:e007
[4]Wu X.BAAS,2009,41:474
[5]Ali Z S,Parsons A R,Zheng H,et al.AJ,2015,809:61
[6]Clarke T E,Kassim N E,Hicks B C,et al.Mm SSI,2011,82:664
[7]Mellema G,Koopmans L V E,Abdalla F A,et al.Ex A,2013,36:235
[8]Cornwell T J,Golap K,Bhatnagar S.IEEE International Conference on Acoustics,Speech,and Signal Processing.Philadelphia:IEEE,2005,5:v/861
[9]Perley R A.Synthesis Imaging in Radio Astronomy II ASP Conference Series,San Francisco:NRAO/NMIMT,1999,180:383.
[10]Cornwell T J,Perley R A.A&A,1992,261:353
[11]Kogan L,Greisen E.AIPS Memo,2009
[12]Cornwell T J,Golap K,Bhatnagar S.IEEE Journal of Selected Topics in Signal Processing,2008,2:647
[13]Humphreys B,Cornwell T.SKA Memo,2011:132
[14]Cornwell T J,Voronkov M A,Humphreys B.ar Xiv:1207.5861
[15]Mc Mullin J P,Waters B,Schiebel D,et al.adass,2007,376:127
[16]Jaeger S.adass,2008,394:623
[17]Offringa A R,Mc Kinley B,Hurley-Walker N,et al.MNRAS,2014,444:606
[18]Vermij E,Fiorin L,Jongerius R,et al.The International Journal of High Performance Computing Applications,2015,29:37
[19]Ord S M,Mitchell D A,Wayth R B,et al.PASP,2010,122:1353
[20]Thompson A R,Moran J W,Swenson G W.Interferometry and Synthesis in Radio Astronomy.New York:Wiley,1986:68-73
[21]Yashar M,Kemball A.Computational Costs of Radio Imaging Algorithms Dealing with the Noncoplanar Baselines Effect:I.TDP Calibration&Processing Group CPG MEMO#3,2009
[22]Dewdney P,Turner W,Braun R,et al.Document number SKA-TEL-SKO-0000308,2015
[23]Lao B Q,An T,Guo S G,et al.Telecommunication Engineering,2016,56:956