System response functions and S-correction results. Left: comparison of the normalized g_PS1 (dashed line), the g_CSP (dotted line), and BV.5 (solid line) system response functions. Also plotted are seven visual-wavelength spectra of SN 2013gy ranging between −16.1 d and −13.4 d as indicated by the color bar. Right, top panel: S-corrections as a function of phase using observed (dark circles) spectra and Hsiao et al. (2007) template spectra (light circles). The smoothing splines of those S-corrections (red and blue lines for g_CSP and BV.5, respectively) enable us to accurately transform g_CSP-band and BV.5-band natural-system photometry to the g_PS1 natural system at any phase. Right, bottom panel: g_CSP- (red squares) and BV.5-band (blue triangles) natural-system photometry, and the S-corrected versions (green squares and green triangles) transformed to the g_PS1 natural system (offset by +1 mag).