韩国一种新的硬件关联函数：使用KVN观测值的性能评估
A New Hardware Correlator in Korea: Performance Evaluation using KVN observations
摘要：
我们报告的一个新的硬件关联函数在韩国，大田关联函数，由韩国天文学和空间科学研究所（KASI）和日本国家天文台（NAOJ）开发的性能评估结果。我们利用韩国VLBI观测网（KVN）和日本VLBI射电天体测量探索（VERA）进行了22~GHz的VLBI观测，并利用大田关联函数对所获得的数据进行了相关。为了评估新的硬件关联函数的性能，我们比较了从大田关联函数器的KVN观测与软件关联函数器，分布式FX（DiFX）的相关输出。我们研究了河外致密射电源的相关通量密度和亮度分布。两个关联函数输出的比较表明，在$<8\%$范围内，两个相关器输出是一致的，这与22~GHz KVN观测的幅度校准不确定度相当。我们还发现，8%的通量密度差异主要是由（a）DiFX软件关联函数和大田硬件关联函数之间条纹相位跟踪方式的差异以及（b）大田关联函数输出的振幅相关的异常图案（双层图案）引起的。双层波型的可见度幅值损失最小为3%。我们的结论是，新的硬件关联函数产生合理的相关输出连续观测，这是一致的输出从DiFX软件关联函数。
英文摘要：
We report results of the performance evaluation of a new hardware correlator in Korea, the Daejeon correlator, developed by the Korea Astronomy and Space Science Institute (KASI) and the National Astronomical Observatory of Japan (NAOJ). We conducted Very Long Baseline Interferometry (VLBI) observations at 22~GHz with the Korean VLBI Network (KVN) in Korea and the VLBI Exploration of Radio Astrometry (VERA) in Japan, and correlated the aquired data with the Daejeon correlator. For evaluating the performance of the new hardware correlator, we compared the correlation outputs from the Daejeon correlator for KVN observations with those from a software correlator, the Distributed FX (DiFX). We investigated the correlated flux densities and brightness distributions of extragalactic compact radio sources. The comparison of the two correlator outputs show that they are consistent with each other within $<8\%$, which is comparable with the amplitude calibration uncertainties of KVN observations at 22~GHz. We also found that the 8\% difference in flux density is caused mainly by (a) the difference in the way of fringe phase tracking between the DiFX software correlator and the Daejeon hardware correlator, and (b) an unusual pattern (a double-layer pattern) of the amplitude correlation output from the Daejeon correlator. The visibility amplitude loss by the double-layer pattern is as small as 3\%. We conclude that the new hardware correlator produces reasonable correlation outputs for continuum observations, which are consistent with the outputs from the DiFX software correlator.
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