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射电望远镜高精度指向测量与大气折射改正
Alternative TitleRadio Telescope Pointing Corrections and Atmospheric Refraction Effect
肖明
Subtype硕士
Thesis Advisor王娜
2015
Degree Grantor中国科学院大学
Place of Conferral中国科学院新疆天文台
Degree Discipline理学
Keyword射电望远镜 望远镜结构形变 大气折射 指向偏差 指向改正
Abstract对于射电望远镜而言,灵敏度和分辨率是衡量其工作性能的两个重要指标。其中,分辨率与射电望远镜口径成正比,即望远镜的口径越大,其分辨率越高。另一方面,望远镜的分辨率与其指向精度的要求也是一致的。射电望远镜的实际工作时的指向位置与其根据位置偏码器输出的位置信息在某种程度上存在一定偏差。影响射电望远镜指向精度的因素主要分为两大类:第一大类,包括某些随机因素(时变的),比如:风载荷和温度载荷。第二大类,包括某些非随机因素(非时变的),比如重力载荷和天线结构设计误差。 射电望远镜的扫描观测方式其实质就是在望远镜对标准射电源跟踪轨迹上叠加了一种均匀且对称的谐波运动。此扫描方法主要是通过实际测量来计算出该天体的在天区中的真实位置坐标,主要原理是利用射电望远镜在做跟踪扫描运动时所接受到的来自射电源的辐射功率变化,进而计算和判断其指向偏差。本论文主要讨论和分析三种扫描方法(圆锥扫描、李萨如扫描、梅花形扫描)的数学表示形式和实际性能分析。 由于硬件校准的方法并不能完全解决望远镜的指向偏差问题,其所剩余的残差可以通过软件校准的方式来完成。这类改正参数最终需要拟合到射电望远镜的相关控制程序中,从而实现对望远镜指向残差的进一步改正。通过对全天区内的若干标准射电天体进行扫描测量,而获得望远镜在全天区范围内指向残差的大样本数据及分布情况,进而针对该大样本数据进行统计计算和软件拟合来完成最终的指向偏差改正。 一般的,大气折射对望远镜指向偏差的影响需要单独考虑,本论文针对地球中性大气的物理属性和大气折射对射电望远镜指向的影响进行模型分析和计算,以改进型的“三段式”标准指向改正模型为基础,重点分析了由大气折射所造成的高阶指向误差的改正方案。在此模型基础上,模拟新疆天文台南山观测基地的实际气候特征,对该方案进行了验证。通过分析和比较这些结果与射电望远镜指向精度的要求,进而为将来的大口径射电望远镜高精度指向修正提出了参考方案。
Other AbstractFor an optimal performance of the reflector antenna, it is important that the system of the hardware is properly focused. For a prime focus reflector this means localizing the phase center of the feed in the focus of the primary antenna. In this essay we, on the first hand, discuss the influence of a defocus, either axial or lateral, on the pointing characteristics, and then, describe the method to establish the overall pointing model of the antenna. On the other hand, the object trajectory (its position versus time) is typically known with high accuracy, and this trajectory is programmed into the antenna’s software, forming the antenna command. However, due to some environmental disturbances such as temperature gradient, wind and gravity load, and somewhat manufacturing imperfections, the antenna does not point precisely towards the object. Scanning techniques are used to determine the object position. Scanning movements of an antenna are small harmonic axial movements added to the antenna trajectory. The scanning motion produces power variations of the received signal, which are used to estimate the positions of radio sources. Three different scanning patterns, conical scan, Lissajous scan, and rosette scan, are presented and analyzed here. The neutral and ionized media lying between a celestial object and the surface of the earth often have profound important effects on the radiation fields traversing them. The most important of these media are the neutral lower atmosphere. The large-scale structures in the media give rise to refractive effects. These effects, which can be analyzed in terms of geometric optics and Fermat’s principle, are the deflection of the radio waves, the change of the propagation velocity, and the rotation of the polarization plane. Accurate refractive corrections, which depend on astronomical pointing measurements, are required by observations using large aperture radio telescopes, especially at higher frequency bands. This essay also presents the advanced model for atmospheric refraction corrections and shows the improvements compared to traditional methods, particularly at low elevations. In addition, real-time refraction corrections, which involve the higher-orders refraction corrections of telescope pointing errors, for example, the effect of uneven temperature distribution over a wide scale range, or the differential refraction across a large aperture telescopes, are also analyzed in this essay.
Subject Area天体物理
Language中文
Document Type学位论文
Identifierhttp://ir.xao.ac.cn/handle/45760611-7/647
Collection研究生学位论文
射电天文研究室
Recommended Citation
GB/T 7714
肖明. 射电望远镜高精度指向测量与大气折射改正[D]. 中国科学院新疆天文台. 中国科学院大学,2015.
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