密排21厘米阵列观测中初级束非冗余的方向图
Patterns of primary beam non-redundancy in close-packed 21 cm array observations
摘要：
像HERA这样的无线电干涉仪阵列由许多紧密排列的碟形天线组成，排列成规则的图案，从而产生大量长度和方向相同的“冗余”基线。由于相同的基线应该看到相同的天空信号，这提供了一种在不需要显式天空模型的情况下找到相对增益/带通校准的方法。实际上，基线不完全相同的原因有很多，如果不考虑的话，会产生一系列影响，破坏阵列的冗余性，并在校准解决方案中引起虚假结构。在本文中，我们试图建立天线之间的主波束响应的差异如何影响冗余校准的干涉vibrations和由此产生的频率（延迟空间）功率谱的理解。我们使用模拟来研究几种通用类型的主波束的变化，包括主瓣的宽度，旁瓣的角度和频率结构的差异，以及波束的椭圆度和方向。对于所有这些类型，我们发现，额外的时间结构诱导的增益解决方案，特别是当亮点源通过光束。相比之下，只有一个低水平的额外的光谱结构诱导。时间结构调制宇宙学的21厘米功率谱，但在我们的模拟中只有百分之几的水平。我们还调查了由于退相干效应的信号损失的可能性时，非冗余vibrations平均在一起，发现的退相干是最坏的亮点源通过光束时，其幅度变化显着基线组和类型的主要光束变化。然而，冗余校准吸收了一些退相干效应，与完全校准的情况相比，减少了其影响。
英文摘要：
Radio interferometer arrays such as HERA consist of many close-packed dishes arranged in a regular pattern, giving rise to a large number of `redundant' baselines with the same length and orientation. Since identical baselines should see an identical sky signal, this provides a way of finding a relative gain/bandpass calibration without needing an explicit sky model. In reality, there are many reasons why baselines will not be exactly identical, giving rise to a host of effects that spoil the redundancy of the array and induce spurious structure in the calibration solutions if not accounted for. In this paper, we seek to build an understanding of how differences in the primary beam response between antennas affect redundantly-calibrated interferometric visibilities and their resulting frequency (delay-space) power spectra. We use simulations to study several generic types of primary beam variation, including differences in the width of the main lobe, the angular and frequency structure of the sidelobes, and the beam ellipticity and orientation. For all of these types, we find that additional temporal structure is induced in the gain solutions, particularly when bright point sources pass through the beam. In comparison, only a low level of additional spectral structure is induced. The temporal structure modulates the cosmological 21cm power spectrum, but only at the level of a few percent in our simulations. We also investigate the possibility of signal loss due to decoherence effects when non-redundant visibilities are averaged together, finding that the decoherence is worst when bright point sources pass through the beam, and that its magnitude varies significantly between baseline groups and types of primary beam variation. Redundant calibration absorbs some of the decoherence effect however, reducing its impact compared to if the visibilities were perfectly calibrated.
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