[论文解读] The Standing Wave Phenomenon in Radio Telescopes; Frequency Modulation of the WSRT Primary Beam

Attila Popping, Róbert Braun|ArXiv.org|Dec 14, 2007

Radio Astronomy Observations and Technology参考文献 9被引用 28

一句话总结

本文利用对3C147的全极化全波束全息观测，提出了一种频率分辨的西伯尔克综合孔径射电望远镜（WSRT）主波束经验模型，揭示了在约17 MHz周期下由于驻波效应导致的波束特性中出现强5–10%的幅度调制，如主波束面积、旁瓣比和有效孔径等。该模型通过校正频率相关的波束畸变，显著提升了干涉成像的图像保真度。

ABSTRACT

Inadequacies in the knowledge of the primary beam response of current interferometric arrays often form a limitation to the image fidelity. We hope to overcome these limitations by constructing a frequency-resolved, full-polarization empirical model for the primary beam of the Westerbork Synthesis Radio Telescope (WSRT). Holographic observations, sampling angular scales between about 5 arcmin and 11 degrees, were obtained of a bright compact source (3C147). These permitted measurement of voltage response patterns for seven of the fourteen telescopes in the array and allowed calculation of the mean cross-correlated power beam. Good sampling of the main-lobe, near-in, and far-side-lobes out to a radius of more than 5 degrees was obtained. A robust empirical beam model was detemined in all polarization products and at frequencies between 1322 and 1457 MHz with 1 MHz resolution. Substantial departures from axi-symmetry are apparent in the main-lobe as well as systematic differences between the polarization properties. Surprisingly, many beam properties are modulated at the 5 to 10% level with changing frequency. These include: (1) the main beam area, (2) the side-lobe to main-lobe power ratio, and (3) the effective telescope aperture. These semi-sinusoidsal modulations have a basic period of about 17 MHz, consistent with the natural 'standing wave' period of a 8.75 m focal distance. The deduced frequency modulations of the beam pattern were verified in an independent long duration observation using compact continuum sources at very large off-axis distances. Application of our frequency-resolved beam model should enable higher dynamic range and improved image fidelity for interferometric observations in complex fields. (abridged)

研究动机与目标

• 为解决因对WSRT主波束响应知识不准确而导致的干涉成像保真度下降问题。
• 开发一种高分辨率、全极化的经验波束模型，以捕捉非轴对称及频率相关的波束特征。
• 识别并量化由望远镜结构引起的多路径干涉导致的波束响应中的驻波现象。
• 利用赤道架设的WSRT进行长时间、非轴向连续源观测，验证模型预测。
• 提出一种物理解决方案——全频段各向同性表面处理，以抑制总功率观测中的驻波效应。

提出的方法

• 利用明亮紧凑射电源3C147的全息观测，采样从5角分到11度角尺度的电压响应图案。
• 测量了14座WSRT望远镜中的7座，实现了在离轴最多5度范围内具有高动态范围的平均互相关功率波束计算。
• 在1322至1457 MHz频率范围内，以1 MHz频率分辨率，为所有四种极化分量（XX、XY、YX、YY）构建了经验波束模型。
• 识别出频率相关的波束调制，并将其与8.75米焦距天线的理论驻波周期17 MHz相联系。
• 通过在大离轴角处对紧凑源进行长时间独立观测，验证了预测的流量密度频率调制。
• 提出一种表面处理设计：在白噪声场中应用-2次幂律锥形，以产生全频段各向同性散射，最大限度减少相干反射。

实验结果

研究问题

• RQ1 主波束响应中的频率相关调制如何影响WSRT的干涉成像保真度？
• RQ2 波束中的非轴对称特征和旁瓣与标准cos⁶(rν)近似相比偏离程度如何？
• RQ3 观测到的波束特性中17 MHz周期性是否可归因于望远镜结构中相干反射引起的驻波效应？
• RQ4 该经验波束模型能否有效校正长时间干涉观测中离轴流量密度的变化？
• RQ5 全频段各向同性表面处理能否减轻总功率观测中的驻波效应？

主要发现

• 在1322–1457 MHz频段内，WSRT主波束在主波束面积、旁瓣与主瓣功率比以及有效孔径方面表现出强5–10%的幅度调制。
• 这些调制具有约17 MHz的基本周期，与8.75米焦距天线的理论驻波周期完全一致。
• 该模型成功校正了长时间干涉观测中离轴源的频率相关流量密度变化。
• 模型揭示了显著的非轴对称性以及系统性的极化依赖波束差异，尤其在旁瓣区域表现明显。
• 通过离轴连续源的独立验证，确认了预测的频率相关流量调制，验证了经验模型的可靠性。
• 设计了一种原型全频段各向同性表面处理，采用在白噪声场中施加-2次幂律锥形，有效波长范围从70厘米至1.5厘米，可有效抑制相干反射。