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 (EWHα) is derived using two- and simplified three-filter methods. Synthetic photometry of CALSPEC stars demonstrates the dependency of EWHα 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 EWHα 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 EWHα consistent with high-resolution spectroscopic results.

Introduction
Line Flux Calculation Using Broadband Data
    Instruments
    Equation for Line Flux
        Two-Filter Method
        Simplified Three-Filter Method
    Zero Point for Scientific Unit Conversion
    Systematic Error of Two- and Three-Filter Methods
Validation Using Standard Stars
    Observation and Data Reduction
    Validation via Zero Point
Application to V960 Mon
Discussion and Conclusion
Acknowledgments
Modified Standard Calibration Equation for Broadband Data
References

• Hojae Ahn(School of Space Research, Kyung Hee University, Yongin-si, Gyeonggi-do 17104, Republic of Korea)
• Soojong Pak(School of Space Research, Kyung Hee University, Yongin-si, Gyeonggi-do 17104, Republic of Korea) Corresponding author
• Hyunjin Shim(Department of Earth Science Education, Kyungpook National University, Daegu 41566, Republic of Korea)
• Wonseok Kang(SpaceBeam Inc., Cheongju-si, Chungcheongbuk-do 28165, Republic of Korea)
• Myungshin Im(SNU Astronomy Research Center, Seoul National University, Seoul 08826, Republic of Korea)