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arXiv 1208.6352astro-ph.IM
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了解合成成像动态范围

Understanding synthesis imaging dynamic range

Robert Braun

摘要:

我们开发了一个通用的框架,量化的许多不同的贡献,一个图像的噪声预算与一个阵列的盘子或孔径阵列站。每一个噪音的能见度数据的贡献与相关的相关时间尺度和频率带宽,使一个完整的观察净影响可以评估。所有的参数都是观测频率和能见度基线长度的函数。我们将得到的噪声预算分析,以广泛的现有和计划的望远镜系统,将在约100 MHz和5 GHz之间运行,以确定他们必须克服的校准挑战,以实现热噪声限制性能的幅度。我们的结论是,校准的挑战增加了在几个方面的小尺寸的盘子或孔径阵列站。这将是更具挑战性的实现热噪声限制的性能,使用15米级的菜,而不是目前的阵列的25米菜。一些性能风险可以通过相控阵馈电的部署来缓解,更多的是通过选择(alt,az,pol)安装来缓解,尽管更大的碟形天线直径为风险缓解提供了最佳前景。可以预期成像性能的许多改进是以校准算法的更大复杂性为代价的。然而,一个基本的限制是最终强加的数据约束相对于校准变量的数量不足。即将到来的孔径阵列系统将在以前从未解决过的制度下工作,其中广泛的影响预计将超过热噪声两到三个数量级。利用这些系统实现常规热噪声限制的成像性能提出了极端的挑战。该挑战的大小与孔径阵列站直径成反比。

英文摘要:

We develop a general framework for quantifying the many different contributions to the noise budget of an image made with an array of dishes or aperture array stations. Each noise contribution to the visibility data is associated with a relevant correlation timescale and frequency bandwidth so that the net impact on a complete observation can be assessed. All quantities are parameterised as function of observing frequency and the visibility baseline length. We apply the resulting noise budget analysis to a wide range of existing and planned telescope systems that will operate between about 100 MHz and 5 GHz to ascertain the magnitude of the calibration challenges that they must overcome to achieve thermal noise limited performance. We conclude that calibration challenges are increased in several respects by small dimensions of the dishes or aperture array stations. It will be more challenging to achieve thermal noise limited performance using 15 m class dishes rather than the 25 m dishes of current arrays. Some of the performance risks are mitigated by the deployment of phased array feeds and more with the choice of an (alt,az,pol) mount, although a larger dish diameter offers the best prospects for risk mitigation. Many improvements to imaging performance can be anticipated at the expense of greater complexity in calibration algorithms. However, a fundamental limitation is ultimately imposed by an insufficient number of data constraints relative to calibration variables. The upcoming aperture array systems will be operating in a regime that has never previously been addressed, where a wide range of effects are expected to exceed the thermal noise by two to three orders of magnitude. Achieving routine thermal noise limited imaging performance with these systems presents an extreme challenge. The magnitude of that challenge is inversely related to the aperture array station diameter.

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