Allen Telescope Array polarimetric observation of 3C286
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This dataset contains a polarimetric interferometry observation of the quasar 3C286 done with two dishes from Allen Telescope Array on 2020-10-31. The observation was done during two hours around transit, so it has large parallactic angle coverage. Eight different frequency bands between 1 and 8 GHz were observed in turns. Scans of 100 seconds were done with a frequency change between each scan. The frequency bands observed are 1413 MHz, 1665 MHz, 2675 MHz, 4820 MHz, 6663 MHz, and 8665 MHz. A total of 30.72 MHz of spectrum divided in 256 frequency bins were observed in each band. This data set contains the same data in two formats: the raw measurements produced with GNU Radio, and the observations in UVFITS format. The raw measurements consists of power spectra and cross-spectra for the X and Y linear polarizations of each antenna, using a coherent averaging interval of 0.1 seconds. The observation was done with antennas 1h and 4g from the array, using two USRP N32x to digitize the signals, and GNU Radio to perform the digital signal processing. Both of the antennas were connected to LO d in the RF converter board, which downconverted the signals to a 512 MHz IF. The two channels of each USRP were used to digitize the two polarization channels of each antenna. The USRPs were configured to share the LO between all the channels, and were synchronized to the 10 MHz and 1PPS signals from the telescope timing distribution system. The raw measurements are not phased to the source: instrumental and geometric delays are not corrected in any way. All the raw measurements are stored in GNU Radio Metdata files with detached headers, contained in a flat .tar.xz archive. These files have names such as 3c286_2020-10-31T17:47:32.114581_cross_0. The beginning of the filename indicates the target of the observation, then there is the UTC timestamp corresponding to the beginning of the observation, and finally a code that indicates the measurement channel. Measurement channels are divided in "power" for (real-valued) PSD and "cross" for (complex-valued) cross spectra. The "power" channels are numbered 0 through 3 and correspond to |X1|^2, |Y1|^2, |X2|^2 and |Y2|^2, where Xj and Yj denote the X and Y polarizations of antenna j (here j = 1 corresponds to antenna 1h and j = 2 corresponds to antenna 4g). The "cross" channels are numbered 0 through 5 and correspond to <X1,Y1>, <X1,X2>, <X1,Y2>, <Y1,X2>, <Y1,Y2>, <X2,Y2>. Each file corresponds to one channel and one scan. The metadata detached header contains useful information, such as the precise timestamping of the samples and the observation frequency. Note that some of the files have invalid timestamps in the metadata (they are easy to distinguish because their first timestamp is zero). The timestamp in the filename is only a coarse estimate, and is typically within 1 second error of the precise timestamp in the metadata. The UVFITS files contain only the cross-spectra between different antennas (correlations XX, YY, XY and YX). The data is phased to the source, but the same number of frequency channels and integration interval is maintained. There is one UVFITS file per frequency band. More information about this observation and dataset can be found in the post "Polarimetric observation of 3C286 with Allen Telescope Array" by the first author.
本数据集包含利用艾伦望远镜阵列（Allen Telescope Array）的2台天线，于2020年10月31日对类星体3C286开展的偏振干涉观测数据。本次观测在源中天前后两小时内完成，因此具备充足的视差角覆盖范围。 观测依次覆盖1至8 GHz间的8个不同频段，每次扫描时长为100秒，且每次扫描间切换频段。本次观测的具体频段为1413 MHz、1665 MHz、2675 MHz、4820 MHz、6663 MHz及8665 MHz。每个频段的观测带宽总计30.72 MHz，被划分为256个频率分箱。 本数据集包含两种格式的同源数据：一是由GNU Radio生成的原始测量数据，二是UVFITS格式的观测数据。 原始测量数据包含各天线X、Y线偏振分量的功率谱与互功率谱，采用0.1秒的相干积分时长。本次观测使用阵列中的1h与4g天线，通过2台USRP N32x完成信号数字化，并借助GNU Radio执行数字信号处理。两台天线均连接至射频转换板的LO d端口，该端口将信号下变频至512 MHz中频（IF）。每台USRP的两个通道分别用于数字化单台天线的两个偏振通道。USRP配置为所有通道共享本振，且与望远镜授时系统输出的10 MHz与1PPS信号保持同步。 原始测量数据未针对源进行相位校正，未以任何方式修正仪器延迟与几何延迟。所有原始测量数据存储于分离式头文件的GNU Radio元数据文件中，收纳于扁平化.tar.xz归档文件内。此类文件的命名格式例如：3c286_2020-10-31T17:47:32.114581_cross_0。文件名开头为观测目标，随后为观测开始时刻的UTC时间戳，最后为表示测量通道的编码。 测量通道分为“power”（对应实值功率谱密度）与“cross”（对应复值互功率谱）两类。“power”通道编号为0至3，分别对应|X1|²、|Y1|²、|X2|²与|Y2|²，其中Xj与Yj表示天线j的X、Y偏振分量（此处j=1对应天线1h，j=2对应天线4g）。“cross”通道编号为0至5，分别对应<X1,Y1>、<X1,X2>、<X1,Y2>、<Y1,X2>、<Y1,Y2>、<X2,Y2>。每个文件对应单通道单次扫描的数据。 分离式头文件的元数据包含诸多有用信息，例如采样的精确时间戳与观测频率。需注意，部分文件的元数据中存在无效时间戳（此类文件易于区分，因其首个时间戳为0）。文件名中的时间戳仅为粗略估计值，与元数据中的精确时间戳的误差通常不超过1秒。 UVFITS格式文件仅包含不同天线间的互功率谱相关量（XX、YY、XY与YX相关量）。该数据已针对源完成相位校正，且保留了相同的频率通道数与积分时长。每个频段对应一个UVFITS文件。 更多关于本次观测与本数据集的详细信息，可参阅第一作者发布的博文《利用艾伦望远镜阵列对3C286开展的偏振观测》。
