The Lyot filter is a key device for narrow-band imaging observations of the solar atmosphere, and is widely employed for solar photospheric magnetic field measurements and solar chromospheric imaging observations. The mechanically rotating waveplate speed is around 3-5 s, and the long wavelength switching speed limits the solar atmospheric observation efficiency. If rapid wavelength tuning can be achieved, the effect of turbulent atmosphere on the signal can be somewhat eliminated by smoothing multiple sets of data. The liquid crystal variable retarder is an electronically controlled polarization element, and its utilization for phase instead of rotating waveplates can significantly increase the speed of filter spectral line scanning. There are successful examples of adopting liquid crystals to adjust the transmission band wavelength of Lyot filters. However, the spectral line widths of magnetically sensitive spectral lines leveraged for solar photospheric magnetic field measurements are narrow, typically between 0.1 ? and 0.3 ?. Therefore, a very narrow transmitting band is required for the filter, usually with a FWHM below 0.1 ?. The extremely narrow transmission band puts forward higher demands for the installation, calibration, and control of the filter. Thus, we propose to develop an extremely narrow FWHM liquid crystal filter for solar magnetic field measurements.

We present the development, calibration, and observation verification of a multi-stage liquid crystal Lyot filter with an FWHM of 0.1 ?. First, the optical and electrical control design of the filter is shown. The filter consists of multiple polarizing elements and liquid crystals. Second, the characteristics of the liquid crystal variable retarder are tested by a Mueller matrix spectrometer. The Mueller matrix of the test sample is first obtained, and then the phase retardation and fast axis azimuth parameters are estimated. The time stability, spatial uniformity, and electrical resolution of the liquid crystal variable retarder in the filter are obtained. Then, the filter wavelength is calibrated by a high-resolution solar spectrometer. The FWHM of the filter is measured and the central wavelength of the filter is aligned to the target position. Finally, the liquid crystal filter is placed on the NVST experimental platform for observation and verification. The optical image quality of the filter is measured by a test target, and a clear monochromatic image of the solar photosphere is obtained by fast multi-wavelength observation. The Doppler velocity is calculated by the multi-wavelength monochromatic image.

The filter FWHM is measured to be 0.1 ? with a central wavelength of 5324.19 ? (Fig. 9), and the filter wavelength switching speed is less than 100 ms (Fig. 6). The filter optical image quality meets the imaging requirements of NVST diffraction limit in the NVST solar photospheric narrow-band observation system. The resolution of the liquid crystal filter-based solar narrow-band observation system can reach 0.1284" (Fig. 11). Different atmospheric structures can be found in monochromatic images of different wavelengths of the solar photosphere (Fig. 12). Doppler velocities are calculated using multi-wavelength images, which agree with the HMI results in the quiet region (Fig. 13). The filter employs a pre-filter with a 1 ? blue shift in the central wavelength compared to the target wavelength, and there is leakage risk in the secondary transmission band under the too large offset band. The peak transmittance of the filter can be improved by replacing the polarizer in the Lyot unit at each level with a polarizer with higher transmittance.

We develop a six-stage liquid crystal Lyot filter for solar photospheric magnetic field measurements. This filter has a higher wavelength tuning speed than the conventional filters. The filter has an FWHM of 0.1 ? and can be adopted for scanning observations of the magnetically sensitive spectral lines of the sun's photosphere. The instrumental performance of the liquid crystal filter is verified by observations in the NVST solar photospheric narrow-band observation system. Multi-wavelength high-resolution monochrome images are obtained efficiently. The high-quality image data can be leveraged for quantitative calculation of Doppler velocity and other physical parameters. The measured results show that the spectral line scan speed of the filter is greatly improved compared with that of the conventional filters, and the transmittance half-width and other parameters meet the design requirements. The filter performance has significant features and performance advantages over the conventional filters and meets the high-resolution observation requirements of the solar photospheric magnetic and velocity fields of NVST.