First Demonstration of ECHO: an External Calibrator for Hydrogen Observatories
First Demonstration of ECHO: an External Calibrator for Hydrogen Observatories
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DOI：
10.1088/1538-3873/aa56b9
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发表时间：
2017-02
影响因子：
3.5
通讯作者：
D. Jacobs;Jacob Burba;J. Bowman;A. Neben;Benjamin Stinnett;Lauren Turner;Kali Johnson;M. Busch;Jay Allison;M. Leatham;Victoria Serrano Rodriguez;Mason Denney;David Nelson
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作者：
D. Jacobs;Jacob Burba;J. Bowman;A. Neben;Benjamin Stinnett;Lauren Turner;Kali Johnson;M. Busch;Jay Allison;M. Leatham;Victoria Serrano Rodriguez;Mason Denney;David Nelson
多种仪器正在研究暗能量的限制，观测再电离，并通过探测和表征21厘米氢谱线的红移范围从1到25打开黑暗时代的窗口。这些仪器，包括亚米波段的CHIME和米波段的HERA，是具有多度波束的宽视场阵列，通常在传输模式下工作。准确了解它们的主光束对于从期望的宇宙信号中分离出明亮的前景至关重要，但仅通过天文观测很难实现。以前的低频波束校准工作主要集中在模型验证上，并没有解决21厘米实验对常规波束映射的需求。我们描述了一种无人机安装的校准器的设计和方法，即氢气观测站外部校准器（ECHO），旨在解决这一需求。我们报告了第一批校准137mhz低频偶极子的试验，并将ECHO测量结果与基于Orbcomm卫星星座传输的既定波束映射系统进行了比较。我们以9°的分辨率创建了两个偶极子的波束图，并发现样本噪声范围从天顶的1%到远旁瓣的100%。假设此样本噪声代表测量中的误差，则此范围的较高端还不符合期望的要求，但在Orbcomm上是一种改进。回声回波的总体性能表明，在实践中，通过适度的改进，可以实现所需的精度和角度覆盖。我们确定了测量中系统误差和不确定性的主要来源，并描述了克服它们所需的步骤。
Multiple instruments are pursuing constraints on dark energy, observing reionization and opening a window on the dark ages through the detection and characterization of the 21 cm hydrogen line for redshifts ranging from ∼1 to 25. These instruments, including CHIME in the sub-meter and HERA in the meter bands, are wide-field arrays with multiple-degree beams, typically operating in transit mode. Accurate knowledge of their primary beams is critical for separation of bright foregrounds from the desired cosmological signals, but difficult to achieve through astronomical observations alone. Previous beam calibration work at low frequencies has focused on model verification and does not address the need of 21 cm experiments for routine beam mapping, to the horizon, of the as-built array. We describe the design and methodology of a drone-mounted calibrator, the External Calibrator for Hydrogen Observatories (ECHO), that aims to address this need. We report on a first set of trials to calibrate low-frequency dipoles at 137 MHz and compare ECHO measurements to an established beam-mapping system based on transmissions from the Orbcomm satellite constellation. We create beam maps of two dipoles at a 9° resolution and find sample noise ranging from 1% at the zenith to 100% in the far sidelobes. Assuming this sample noise represents the error in the measurement, the higher end of this range is not yet consistent with the desired requirement but is an improvement on Orbcomm. The overall performance of ECHO suggests that the desired precision and angular coverage is achievable in practice with modest improvements. We identify the main sources of systematic error and uncertainty in our measurements and describe the steps needed to overcome them.