Hα Flux Extraction from Continuum-Dominated Sources Using a Simplified Three-Filter Method

This paper introduces a simple and reliable photometric calibration method to extract Hα line flux from narrowband images. The equivalent width of the Hα line (EW_Hα) is derived using two-and simplified three-filter methods. Synthetic photometry of CALSPEC stars demonstrates the dependency of EW_Hα on the V − R color, described by a skewed Gaussian function within −0.1 < V − R < 0.7. Systematic errors of the two-and three-filter methods are analyzed under 0%–10% R-band flux contamination. Although the three-filter method underestimates EW_Hα by 10%, it exhibits less scatter compared to the two-filter method. The simplified three-filter method was validated with the Landolt SA 107 field and surpasses the two-filter method in terms of precision and accuracy. Additionally, applying our method to V960 Mon yields EW_Hα consistent with high-resolution spectroscopic results.