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One of the essential calibration procedures for the motional Stark effect (MSE) diagnostic system includes obtaining the mapping responses of the angle evaluated from the demodulation of the photoelastic-modulated raw signals in the MSE polarimeter to the incident polarized light either parallel or perpendicular to the Lorentz electric field originated from a flux surface of a tokamak plasma. This in-vessel mapping calibration procedure that is practically very time-consuming and tedious has been improved by a newly developed MSE in-vessel calibration system with the capability of remote and automatic operations for the Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak. The key design has employed the concept that the polarization state is preserved along an observation line of sight once the polarizer surface, or the wave front of the polarized light, is perpendicular to the line of sight. Comparisons with previous manual calibration procedures confirm this concept is applicable practically. The new remote and automatic calibration activities performed with this new system yield unprecedented angular coverage and resolution in the mapping curves between the incident polarization angles and measured angles. The performance of the new calibration mechanism, free from various time constraints related to scheduling tokamak maintenance period, is virtually limited only by the precision of the motorized rotational stage that mounts the polarized light source on it, which is less than a hundredth of a degree. • Utilization of polarization preservation along a line of sight for system compactness. • Unprecedented angular resolutions in the MSE in-vessel mapping calibration. • Flexibility in integrating MSE in-vessel calibration with tokamak maintenance period. [ABSTRACT FROM AUTHOR]