Design, Calibration, and On-Site Validation of an LCVR-Driven Fast-Tunable Lyot Filter for the YOGIS Coronagraph

The Lyot filter, a fundamental element of the Yunnan Observatories Coronagraph Green-line Imaging System (YOGIS) at Lijiang Observatory, utilizes a Liquid Crystal Variable Retarder (LCVR) for swift electrical modulation. This filter allows for precise observations of the coronal green line (Fe XIV, central wavelength 5303 Å) with a narrow full-width at half-maximum (FWHM) of 1 Å and enables rapid adjustment of the transmission band wavelength. This feature aids in capturing the sky background intensity around the green line and images of two line wings (offset by ±0.45 Å from the central wavelength), crucial for determining the green line’s Doppler shift. By employing sky background subtraction and processing line wing images, an improved signal-to-noise ratio (SNR) in coronal green line images is achieved. The YOGIS Lyot filter, an enhancement of the NOrikura Green-line Imaging System (NOGIS) filter, operates at a wavelength of 5303 Å, offers a wavelength tuning range of ±2 Å, and tunes within <60 ms. This study elucidates the filter’s design principles, outlines essential calibration procedures, and validates its performance through on-site observations using the YOGIS.