We characterize the accuracy of linear-polarization mosaics made using the Atacama Large Millimeter/submillimeter Array (ALMA). First, we observed the bright, highly linearly polarized blazar 3C 279 at Bands 3, 5, 6, and 7 (3 mm, 1.6 mm, 1.3 mm, and 0.87 mm, respectively). At each band, we measured the blazar's polarization on an 11$\times$11 grid of evenly-spaced offset pointings covering the full-width at half-maximum (FWHM) area of the primary beam. After applying calibration solutions derived from the on-axis pointing of 3C 279 to all of the on- and off-axis data, we find that the residual polarization errors across the primary beam are similar at all frequencies: the residual errors in linear polarization fraction $P_\textrm{frac}$ and polarization position angle $\chi$ are $\lesssim$0.001 ($\lesssim$0.1% of Stokes $I$) and $\lesssim$1$^\circ$ near the center of the primary beam; the errors increase to $\sim$0.003-0.005 ($\sim$0.3-0.5% of Stokes $I$) and $\sim$1-5$^\circ$ near the FWHM as a result of the asymmetric beam patterns in the (linearly polarized) $Q$ and $U$ maps. We see the expected double-lobed "beam squint" pattern in the circular polarization (Stokes $V$) maps. Second, to test the polarization accuracy in a typical ALMA project, we performed observations of continuum linear polarization toward the Kleinmann-Low nebula in Orion (Orion-KL) using several mosaic patterns at Bands 3 and 6. We show that after mosaicking, the residual off-axis errors decrease as a result of overlapping multiple pointings. Finally, we compare the ALMA mosaics with an archival 1.3 mm CARMA polarization mosaic of Orion-KL and find good consistency in the polarization patterns.

中文翻译：

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表征 ALMA 线偏振马赛克的精度

我们描述了使用阿塔卡马大型毫米/亚毫米阵列 (ALMA) 制作的线性偏振马赛克的准确性。首先，我们在波段 3、5、6 和 7（分别为 3 毫米、1.6 毫米、1.3 毫米和 0.87 毫米）处观察到明亮的、高度线性偏振的耀变体 3C 279。在每个波段，我们在覆盖主光束半高全宽 (FWHM) 区域的均匀间隔偏移指向的 11$\times$11 网格上测量了耀变体的极化。将来自 3C 279 轴上指向的校准解决方案应用于所有轴上和轴外数据后，我们发现主光束的残余偏振误差在所有频率下都相似：线性偏振部分的残余误差$P_\textrm{frac}$ 和极化位置角 $\chi$ 为 $\lesssim$0.001 ($\lesssim$0. 1% 的斯托克斯 $I$) 和 $\lesssim$1$^\circ$ 靠近主光束的中心；误差增加到 $\sim$0.003-0.005（$\sim$0.3-0.5% 的 Stokes $I$）和 $\sim$1-5$^\circ$ 在 FWHM 附近由于（线性极化）$Q$ 和 $U$ 地图。我们在圆偏振 (Stokes $V$) 图中看到了预期的双瓣“光束斜视”模式。其次，为了测试典型 ALMA 项目中的偏振精度，我们使用波段 3 和波段 6 的几种镶嵌图案对猎户座中的克莱曼-低星云 (Orion-KL) 的连续线性偏振进行了观测。我们表明，镶嵌后，由于重叠多个指向，残余离轴误差会减少。最后，我们将 ALMA 马赛克与档案 1 进行比较。