用WIRC +Pol实现近红外偏振光谱测量精度优于0.1%
Achieving a Spectropolarimetric Precision Better than 0.1% in the Near-Infrared with WIRC+Pol
摘要：
WIRC+Pol是Palomar天文台200英寸望远镜上的近红外低分辨率分光偏振仪。该仪器利用偏振光栅同时进行偏振分束和光谱色散。它可以使用焦平面狭缝来减少天空背景，也可以使用无狭缝模式。四个不同的光谱采样四个线性偏振角记录在焦平面，使仪器能够测量所有的线性偏振状态在一个曝光。该仪器自2017年2月以来一直在天空中运行，我们发现可能由平坦场和重力对仪器的影响引起的系统误差将我们的准确度限制在1%左右。这些系统效应是缓慢变化的，因此可以用偏振调制器消除。2019年3月，在WIRC+Pol前面安装了半波片调制器和线性偏振器。调制器按预期工作，使我们能够测量和消除我们以前观察到的所有仪器偏振。最深的积分亮点源（J = 7.689，非偏振星HD 65970）表现出的不确定性在q和u的0.03%，每个光谱通道，符合光子噪声限制。对较暗光源的观测表明，该仪器可以达到无缝模式下观测的光子噪声极限。对于狭缝中的观测，不确定性仍然是光子噪声极限以上的一个因素，可能是由于狭缝损失。
英文摘要：
WIRC+Pol is a near-infrared low-resolution spectropolarimeter on the 200-inch Telescope at Palomar Observatory. The instrument utilizes a polarization grating to perform polarimetric beam splitting and spectral dispersion simultaneously. It can operate either with a focal plane slit to reduce sky background or in a slitless mode. Four different spectra sampling four linear polarization angles are recorded in the focal plane, allowing the instrument to measure all linear polarization states in one exposure. The instrument has been on-sky since February 2017 and we found that the systematic errors, likely arising from flat fielding and gravity effects on the instrument, limit our accuracy to ~1%. These systematic effects were slowly varying, and hence could be removed with a polarimetric modulator. A half-wave plate modulator and a linear polarizer were installed in front of WIRC+Pol in March 2019. The modulator worked as expected, allowing us to measure and remove all instrumental polarization we previously observed. The deepest integration on a bright point source (J = 7.689, unpolarized star HD 65970) demonstrated uncertainties in q and u of 0.03% per spectral channel, consistent with the photon noise limit. Observations of fainter sources showed that the instrument could reach the photon noise limit for observations in the slitless mode. For observations in slit, the uncertainties were still a factor of few above the photon noise limit, likely due to slit loss.
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