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arXiv 0807.2255astro-ph
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亚毫米波阵列偏振计

The Submillimeter Array Polarimeter

Daniel P. Marrone, Ramprasad Rao

摘要:

我们描述了亚毫米阵列(SMA)偏振计,偏振转换器和饲料复用器安装在SMA。偏振计使用窄带四分之一波片从线偏振SMA馈源产生圆偏振灵敏度。波片安装在计算机控制下的旋转台中,使得每个天线的极化旋向性可以快速选择。波片的定位被发现是高度可重复的,优于0.2度。虽然在任何时候都只检测到单个极化,但是通过沃尔什函数模式中的天线极化的协调切换,在每个基线上对左圆极化和右圆极化的所有四个互相关进行有效采样。最初的一组抗反射涂层石英和蓝宝石波片允许在345 GHz附近进行偏振测量;这些波片已被用于325和350 GHz之间的观测。每个天线的频率相关的交叉极化,主要是由于单元件波片的延迟相位随频率的变化,可以通过观察亮点源精确地测量。这样的测量表明,每个天线的交叉极化是百分之几或更小的和稳定的,符合预期的频率依赖性和非常小的对准误差。旋光仪现在可作为SMA的设施仪器供一般使用。

英文摘要:

We describe the Submillimeter Array (SMA) Polarimeter, a polarization converter and feed multiplexer installed on the SMA. The polarimeter uses narrow-band quarter-wave plates to generate circular polarization sensitivity from the linearly-polarized SMA feeds. The wave plates are mounted in rotation stages under computer control so that the polarization handedness of each antenna is rapidly selectable. Positioning of the wave plates is found to be highly repeatable, better than 0.2 degrees. Although only a single polarization is detected at any time, all four cross correlations of left- and right-circular polarization are efficiently sampled on each baseline through coordinated switching of the antenna polarizations in Walsh function patterns. The initial set of anti-reflection coated quartz and sapphire wave plates allows polarimetry near 345 GHz; these plates have been have been used in observations between 325 and 350 GHz. The frequency-dependent cross-polarization of each antenna, largely due to the variation with frequency of the retardation phase of the single-element wave plates, can be measured precisely through observations of bright point sources. Such measurements indicate that the cross-polarization of each antenna is a few percent or smaller and stable, consistent with the expected frequency dependence and very small alignment errors. The polarimeter is now available for general use as a facility instrument of the SMA.

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