Model spectrum of reflected light for 51 Pegasi b based on a continuum normalised synthetic spectrum of its host star, at the spectral resolution of HARPS-N (F_{mod, norm}, R = 115 000, blue line in left-hand panel). The black solid line shows the effect of broadening the model to v_refl = 12.0 km s⁻¹ via convolution with the rotation kernel shown in the right-hand panel. It is approximately equivalent to R ~ 25 000. Many of the narrow spectral features are blended and shallower in the broadened model, which reduces the last term of Eq. (1) and lowers S/N_p.