ALMA long baseline phase calibration using phase referencing
ALMA long baseline phase calibration using phase referencing
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使用相位参考的 ALMA 长基线相位校准
DOI：
10.1117/12.2232301
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发表时间：
2016
期刊：
影响因子：
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通讯作者：
Catherine Vlahakis
中科院分区：
文献类型：
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作者：
Yoshiharu Asaki;Satoki Matsushita;Edward B. Fomalont;Stuartt A. Corder;Lars-Ake Nyman;William R. F. Dent;Neil M. Philips;Akihiko Hirota;Satoko Takahashi;Baltasar Vila-Vilaro;Bojan Nikolic;Todd R. Hunter;Anthony Remijan;Catherine Vlahakis
阿塔卡马大型毫米/亚毫米阵列（阿尔马）是世界上最大的毫米/亚毫米望远镜，提供了前所未有的灵敏度和空间分辨率。为了获得最高的成像能力，长基线的干涉相位校准是最重要的课题之一：基线越长，由于地球大气的湍流运动，特别是对流层中的水汽，相位稳定性变得越差。为了克服这个问题，阿尔马采用了相位校正方案，使用水蒸气辐射计（WVR）来估计水蒸气含量的量沿着天线视线。另一种技术是相位参考，通过快速改变天线指向，轮流观察科学目标和附近的校准器。我们对WVR相位校正和天线快速切换（FS）相位参考的混合技术进行了可行性研究（WVR+FS相位校正），对于阿尔马16 km最长基线，在以下情况下：（1）目标和校准器使用相同的观测频率，以及（2）目标和校准器分别使用较高和较低的频率，典型的开关周期时间为20秒。发现混合技术的相位校正性能是有希望的，其中附近的校准器位于距离科学目标大约3 μ m的范围内，并且与120 s切换周期时间相比，具有20 s切换周期时间的相位校正显著地改善了具有上述分离角标准的性能。目前试验的相位校准方法显示出与观测频率无关的相同性能。这一结果对于更高频率的观测尤其重要，因为很难找到一个接近任意天空位置的明亮校准器。在一系列的实验中还发现，影响图像质量的相位误差不仅来自对流层低层的水汽含量，而且来自大气的一个大结构，其典型的单元尺度为几十公里。
The Atacama Large Millimeter/submillimeter Array (ALMA) is the world's largest millimeter/submillimeter telescope and provides unprecedented sensitivities and spatial resolutions. To achieve the highest imaging capabilities, interferometric phase calibration for the long baselines is one of the most important subjects: The longer the baselines, the worse the phase stability becomes because of turbulent motions of the Earth's atmosphere, especially, the water vapor in the troposphere. To overcome this subject, ALMA adopts a phase correction scheme using a Water Vapor Radiometer (WVR) to estimate the amount of water vapor content along the antenna line of sight. An additional technique is phase referencing, in which a science target and a nearby calibrator are observed by turn by quickly changing the antenna pointing. We conducted feasibility studies of the hybrid technique with the WVR phase correction and the antenna Fast Switching (FS) phase referencing (WVR+FS phase correction) for the ALMA 16 km longest baselines in cases that (1) the same observing frequency both for a target and calibrator is used, and (2) higher and lower frequencies for a target and calibrator, respectively, with a typical switching cycle time of 20 s. It was found that the phase correction performance of the hybrid technique is promising where a nearby calibrator is located within roughly 3◦ from a science target, and that the phase correction with 20 s switching cycle time significantly improves the performance with the above separation angle criterion comparing to the 120 s switching cycle time. The currently trial phase calibration method shows the same performance independent of the observing frequencies. This result is especially important for the higher frequency observations because it becomes difficult to find a bright calibrator close to an arbitrary sky position. In the series of our experiments, it is also found that phase errors affecting the image quality come from not only the water vapor content in the lower troposphere but also a large structure of the atmosphere with a typical cell scale of a few tens of kilometers.