The upper panel presents the best-fit hydrogen-dominated model spectrum (blue line) with 1σ model uncertainties (computed from the parameter errors; blue shadow) for the SED of the white dwarf primary COCONUTS-1A. The observed fluxes and uncertainties from Gaia DR2, PS1, UHS, and 2MASS are shown as error bars, and the 2MASS photometry are plotted in gray since they are not included in our SED analysis (Section 4). The synthesized broadband fluxes using the best-fit model spectrum are consistent with observations and are shown as blue circles (solid circles for Gaia DR2, PS1, and UHS, and open circles for 2MASS). The lower panel compares the best-fit model spectrum (blue) with the flux-calibrated UH 2.2 m/SNIFS spectrum (black) and flux uncertainties (gray shadow) near the Hα line. The model spectrum is convolved at the SNIFS resolution. We note a possible spectral feature around 6540 Å, but a higher S/N is needed to assess the reality of this feature. The agreement between the SED-derived model and the observed Hα line confirms the hydrogen-dominated atmosphere of COCONUTS-1A.