Aditya-L1机载分光偏振仪：偏振调制和解调
Spectropolarimeter on-board the Aditya-L1: Polarization Modulation and Demodulation
摘要：
Aditya-L1飞行任务所载可见发射线日冕仪有效载荷的主要科学目标之一是绘制日冕磁场拓扑图，并定期定量估计纵向磁场。VELC的红外通道配备了偏振计，在1074.7nm处对Fe XIII线进行全斯托克斯光谱偏振观测。偏振计采用双光束结构，波片连续旋转作为偏振调制器。在Fe~{ \sc xiii}谱线中，由于塞曼效应引起的圆偏振和由于饱和汉勒效应引起的磁场存在下的线偏振退偏的检测需要高的信噪比（SNR）。由于光子数量有限，预计需要较长的积分时间才能建立所需的SNR。换句话说，来自大量调制周期的信号将被平均以实现所需的SNR。这造成了若干困难。其中之一是数据量的增加，另一个是连续调制周期中调制矩阵的变化。对于VELC分光偏振计（VELC/SP）来说，后一种效应是由于延迟器的旋转周期与信号检测时间长度之间的不匹配而产生的。本文表明，通过适当地选择每半旋转的样本数，可以优化数据量。一个潜在的解决方案，建议占调制矩阵的变化，从一个周期到另一个。
英文摘要：
One of the major science goals of the Visible Emission Line Coronagraph (VELC) payload aboard the Aditya-L1 mission is to map the coronal magnetic field topology and the quantitative estimation of longitudinal magnetic field on routine basis. The infrared (IR) channel of VELC is equipped with a polarimeter to carry out full Stokes spectropolarimetric observations in the Fe XIII line at 1074.7~nm. The polarimeter is in dual-beam setup with continuously rotating waveplate as the polarization modulator. Detection of circular polarization due to Zeeman effect and depolarization of linear polarization in the presence of magnetic field due to saturated Hanle effect in the Fe~{\sc xiii} line require high signal-to-noise ratio (SNR). Due to limited number of photons, long integration times are expected to build the required SNR. In other words signal from a large number of modulation cycles are to be averaged to achieve the required SNR. This poses several difficulties. One of them is the increase in data volume and the other one is the change in modulation matrix in successive modulation cycles. The latter effect arises due to a mismatch between the retarder's rotation period and the length of the signal detection time in the case of VELC spectropolarimeter (VELC/SP). It is shown in this paper that by appropriately choosing the number of samples per half rotation the data volume can be optimized. A potential solution is suggested to account for modulation matrix variation from one cycle to the other.
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