Application of the tunable narrowband filter in measurements of solar magnetic fields

Solar magnetic field observations achieve a precise understanding of the solar activity. Imaging spectropolarimetry obtains the magnetic field strength for the entire field of view simultaneously and spectral samples sequentially. By employing high-resolution reconstruction methods, ground-based solar imaging spectropolarimeters are suitable for magnetic field measurements with a high spatial and temporal resolution. The space-borne solar imaging spectropolarimeter has a compact optical path, stable performance, and convenient data processing. Furthermore, it can be used for deep space exploration. Narrowband tunable filters are the primary spectroscopic components of imaging spectropolarimeters for obtaining narrowband light, with various structural forms for different application scenarios from ground to space. Tunable narrowband filters are classified into two categories according to different principles: The Fabry-Perot filter (F-P filter) and the polarization interference filter. The Fabry-Perot filter is divided into two types based on wavelength adjustment methods: Air cavity and lithium niobate. Polarization interference filters are divided into Lyot filters and solid Michelson interferometers according to different beam-splitting structures. Magnetic field measurement using solar imaging spectropolarimetry is a high-precision measurement with strong noise and weak signal. The performance of the narrowband tunable filter determines the accuracy of the spectral transmittance curve, and the filter instability will increase the noise and affect the accuracy of the magnetic field intensity. The narrowband tunable filter, as the most complex component in the optical path of imaging spectropolarimeters, must solve different technical problems in various application scenarios, and the key technology directly affects whether the magnetic image can be obtained. Therefore, how to design the filter system based on different application scenarios is a problem that must be solved for imaging spectropolarimetry to achieve high-precision observation. This paper investigates famous solar spectropolarimeters of ground-based, balloon-borne, and space-borne. Two or three types of filters are used to meet the requirements of different application scenarios. Air-gap F-P filters are characterized by high transmittance, high spectral resolution, and low sidelobe, and they can be applied in the near-infrared band. Its most remarkable advantage is that larger apertures can be manufactured as production technology advances. The stability of this filter is the toughest problem. The filter relies on a calibrated optical path to achieve accurate adjustment even when used on the Coude platform. As a result, the F-P filter requires a relatively stable platform with a large space. It could have a sizable aperture, making it suitable for the next generation of large-aperture ground-based telescopes with a Coude platform. LiNbO3 F-P filter, solid Michelson interferometer, and Lyot filter have the advantage of stable performance. They are suitable for space-borne, balloon-borne, and other systems with poor mechanical stability. The Lyot filter has excellent stability. Temperature is the only factor that needs to be considered. The surface precision requirements of the Lyot filter are low, whereas those of the solid Michelson and LiNbO3 F-P filters are high. The LiNbO3 F-P filter needs to consider the influence of high voltage. The Lyot filter is made of natural crystal, which makes achieving a large aperture difficult. The artificial glass or crystal used in the solid Michelson interferometer and LiNbO3 F-P filter allows for a larger aperture but places high demands on processing technology. One of China's 8 m CGST's scientific goals is to achieve high-precision and high-resolution vector magnetic field measurements in NIR, which requires further development of a large-aperture air-gap F-P filter in NIR. China has developed several 1 m aperture solar telescopes in recent years. The 2 m RST of Yunnan Observatory will be completed soon. RST's imaging spectropolarimeter is located in the main focus of the telescope, which requires stable filter performance, and the Lyot filter meets the demand. China's next-generation space solar magnetometer can succeed in choosing the Lyot filter. If the spectral resolution is required, a combination of tunable wide-field solid Michelson interferometer and Lyot filter can be used. LiNbO3 F-P filters are also a new research area. It is foreseen that China will have multiple solar telescopes to achieve high-resolution vector magnetic field measurements in multiple bands and make contributions to solar physics.