A High-Efficiency and High-Accuracy Polarimeter for Solar Magnetic Field Measurements
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A High-Efficiency and High-Accuracy Polarimeter for Solar Magnetic Field Measurements Deqing Ren1
· Zijian Han2,3,4
· Jing Guo2,3
Received: 20 December 2019 / Accepted: 24 July 2020 © Springer Nature B.V. 2020
Abstract Solar activity is dominated by the magnetic field. Nowadays, a polarimeter is a mandatory tool to measure solar magnetic fields, which are generally faint and correspond to a polarization of an order of 10−2 –10−4 . As such, polarization measurements of high efficiency with a high accuracy are crucial to investigate faint magnetic fields. Here we propose a high-efficiency and high-accuracy polarimeter, which is based on a pair of nematic liquid crystal variable retarders (LCVRs) and a Wollaston prism (WP). It uses a dedicated Stokes modulation strategy to achieve high efficiency. A calibration unit (CU) is developed to measure the polarimeter response matrix, which provides a high-precision calibration to correct possible systematic errors. Compared with other traditional polarimeters, the modulation scheme of our polarimeter is flexible. In addition to be able to measure all the three Stokes polarization components (Q, U , or V ) simultaneously, it can also measure one or two of these polarization components alone, with high polarization efficiency. Dedicated alignment and calibration techniques optimized for our polarimeter are developed and high measurement accuracy is achieved. In our laboratory experimental test, our two-image based polarization measurement delivers an overall measurement accuracy of the order of 10−4 , which is about 10 times better, compared with our previous polarimeters that use the traditional four-image polarization modulation. This work provides a new option for high-efficiency and high-accuracy polarization measurement for future solar synoptic observations.
B D. Ren
[email protected] Z. Han [email protected] J. Guo [email protected]
1
Physics & Astronomy Department, California State University Northridge, Northridge 91330, USA
2
National Astronomical Observatories/Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences, Nanjing 210042, China
3
Key Laboratory of Astronomical Optics & Technology, Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences, Nanjing 210042, China
4
University of Chinese Academy of Sciences, Beijing 100049, China
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Keywords Instrumentation and data management · Polarization · Optical · Magnetic fields · Chromosphere
1. Introduction Measurements of solar magnetic fields help us to explore and research the intrinsic properties of the solar atmosphere (Hofmann and Rendtel, 2003). Polarimetry measurements are routinely used to infer the solar magnetic fields through the Zeeman and Hanle effects (Hough, 2005; Asensio Ramos, Trujillo Bueno, and Landi Degl’Innocenti, 2008). With the access to large solar telescopes, polarization signals related with electromagnetic radiation are being measured with the highest possible resolution and accuracy, findin
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