Field-dependent wavelength drift compensation in Fabry–Perot solar imaging spectrometers

Context . Fabry–Perot interferometers (FPIs) have become fundamental instruments for high-resolution imaging spectroscopy and magnetic field measurements in solar physics. In telecentric configurations, cavity defects of the FPI can introduce an inhomogeneous wavelength response across the field of view (FOV), which severely limits the accuracy of velocity and magnetic field diagnostics. Therefore, precise correction of the wavelength shift distribution is essential. Aims . This study presents a method for compensating for the field-dependent wavelength drift of a telecentric air-gapped FPI instrument. The method allows for accurate calibration using data directly recorded by the instrument and provides precise measurements of wavelength shifts under actual observing conditions. This results in a reliable wavelength correction, which serves as a prerequisite for high-precision scientific observations. Methods . The method scanned solar spectral lines directly through the scientific imaging path. Relative wavelength shifts across the FOV were then extracted using a cross-correlation algorithm combined with centroid localization. These shifts were decomposed into linear and nonlinear components. A response model linking actuator offset voltages to the linear drift slopes was constructed to actively compensate for the linear part, while the remaining nonlinear component is characterized as the intrinsic instrument bias. Results . The method was applied to the Visible-light Imaging Spectrometer (VIS) at the New Vacuum Solar Telescope (NVST) of Yunnan Observatories. The voltage-to-slope response model demonstrated excellent performance, with R 2 = 0.998 and root-mean-square error (RMSE) ≤0.007 V for both offset voltages, which enabled effective correction of the linear wavelength drift. After correction, the maximum wavelength shift decreased from 0.14 Å to 0.05 Å, significantly improving the homogeneity of the wavelength response and suppressing linear drift artifacts, thereby revealing previously obscured solar velocity signals. Moreover, a comparative analysis was conducted using the laser scanning technique, and showed excellent agreement with the results obtained from the proposed method.