XPOL -IRAM 30米望远镜上的相关偏振计
XPOL - the correlation polarimeter at the IRAM 30m telescope
摘要：
XPOL是大型毫米望远镜的第一个相关偏振计，它使用灵活的数字相关器同时测量所有四个斯托克斯参数，即总功率I、线偏振分量Q和U以及圆偏振V。后端的多功能性为有效的连续观测提供了足够的带宽，并为窄线观测提供了足够的光谱分辨率（40 kHz）。我们证明，偏振特定的校准处理有足够的精度，特别是天文台的两个正交线偏振接收器之间的相对相位。仪器偏振的许多方面进行了研究，在3 mm波长的所有斯托克斯参数：轴上与点源和离轴光束地图。斯托克斯Q，其被测量为接收器之间的功率差，在1.5%的水平上受仪器偏振的影响。作为互相关测量的斯托克斯U和V受到的影响非常小（最大旁瓣为0.6%（U）和0.3%（V））。这些电平主要取决于接收机对准的精度。当对准最佳（<~ 0.3”）时，它们达到由馈电喇叭的小椭圆度设置的最小水平。低偏振旁瓣的第二个关键先决条件是偏振分束器栅极的正确取向。详细分析了其交叉极化特性。因此，即使对于扩展源，XPOL观测也仅受斯托克斯U和V中的接收器噪声的限制。Stokes Q观测值的系统效应在1.5%水平。通过适当的预防措施，这种限制可以克服点源。扩展源的斯托克斯Q观测是XPOL最困难的。
英文摘要：
XPOL, the first correlation polarimeter at a large millimeter telescope, uses a flexible digital correlator to measure all four Stokes parameters simultaneously, i.e. the total power I, the linear polarization components Q and U, and the circular polarization V. The versatility of the backend provides adequate bandwidth for efficient continuum observations as well as sufficient spectral resolution (40 kHz) for observations of narrow lines. We demonstrate that the polarimetry specific calibrations are handled with sufficient precision, in particular the relative phase between the Observatory's two orthogonally linearly polarized receivers. The many facets of instrumental polarization are studied at 3mm wavelength in all Stokes parameters: on-axis with point sources and off-axis with beam maps. Stokes Q which is measured as the power difference between the receivers is affected by instrumental polarization at the 1.5% level. Stokes U and V which are measured as cross correlations are very little affected (maximum sidelobes 0.6% (U) and 0.3% (V)). These levels critically depend on the precision of the receiver alignment. They reach these minimum levels set by small ellipticities of the feed horns when alignment is optimum (<~ 0.3"). A second critical prerequisite for low polarization sidelobes turned out to be the correct orientation of the polarization splitter grid. Its cross polarization properties are modeled in detail. XPOL observations are therefore limited only by receiver noise in Stokes U and V even for extended sources. Systematic effects set in at the 1.5% level in observations of Stokes Q. With proper precautions, this limitation can be overcome for point sources. Stokes Q observations of extended sources are the most difficult with XPOL.
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