All‐Stokes Parameterization of the Main Beam and First Sidelobe for the Arecibo Radio Telescope
All‐Stokes Parameterization of the Main Beam and First Sidelobe for the Arecibo Radio Telescope
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DOI：
10.1086/323290
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发表时间：
2001-07
影响因子：
3.5
通讯作者：
C. Heiles;P. Perillat;M. Nolan;D. Lorimer;R. Bhat;T. Ghosh;E. Howell;M. Lewis;K. O’Neil;C. Salter;S. Stanimirović
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文献类型：
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作者：
C. Heiles;P. Perillat;M. Nolan;D. Lorimer;R. Bhat;T. Ghosh;E. Howell;M. Lewis;K. O’Neil;C. Salter;S. Stanimirović
扩展发射的射电天文学测量需要波束形状和响应的知识，因为测量需要对诸如波束效率和波束宽度之类的量进行校正。我们描述了一个方案，其特征在于在所有斯托克斯参数的主波束和旁瓣采用参数，允许重建的完整的波束图案，也提供了一个简单的方法来看看如何波束的变化与方位角，天顶角，和时间。对于Stokes I中的主光束，参数包括光束宽度、椭圆率及其方向、彗差及其方向、点源增益和积分增益（或等效的主光束效率）;对于其他Stokes参数，光束参数包括光束偏斜和光束挤压。对于斯托克斯I中的第一个旁瓣环，参数包括描述高度、半径和径向宽度的八项傅立叶级数;对于其他斯托克斯参数，它们只包括旁瓣的分数极化。我们说明的技术，将其应用到阿雷西博望远镜。主波束宽度更小，旁瓣水平高于相同有效面积的均匀照明孔径。这些效应通过阻塞孔径适度良好地建模，其中阻塞面积等于有效面积的约10%（这对应于5%的物理阻塞）。在偏振发射中，光束偏斜（正交偏振之间指向方向的差异）和挤压（正交偏振之间波束宽度的差异）的影响不符合理论预期，并且高于预期;这些影响几乎肯定是由阻塞引起的。第一副瓣由于阻塞而高度极化。这些偏振效应导致亮度温度的空间导数对偏振发射图的严重污染。
Radio astronomical measurements of extended emission require knowledge of the beam shape and response because the measurements need correction for quantities such as beam efficiency and beamwidth. We describe a scheme that characterizes the main beam and sidelobe in all Stokes parameters employing parameters that allow reconstruction of the complete beam patterns and, also, afford an easy way to see how the beam changes with azimuth, zenith angle, and time. For the main beam in Stokes I, the parameters include the beamwidth, ellipticity and its orientation, coma and its orientation, the point‐source gain, and the integrated gain (or, equivalently, the main‐beam efficiency); for the other Stokes parameters, the beam parameters include beam squint and beam squash. For the first sidelobe ring in Stokes I, the parameters include an eight‐term Fourier series describing the height, radius, and radial width; for the other Stokes parameters they include only the sidelobe’s fractional polarization. We illustrate the technique by applying it to the Arecibo telescope. The main‐beam width is smaller and the sidelobe levels higher than for a uniformly illuminated aperture of the same effective area. These effects are modeled modestly well by a blocked aperture, with the blocked area equal to about 10% of the effective area (this corresponds to 5% physical blockage). In polarized emission, the effects of beam squint (difference in pointing direction between orthogonal polarizations) and squash (difference in beamwidth between orthogonal polarizations) do not correspond to theoretical expectation and are higher than expected; these effects are almost certainly caused by the blockage. The first sidelobe is highly polarized because of blockage. These polarization effects lead to severe contamination of maps of polarized emission by spatial derivatives in brightness temperature.