Examples of predicted phase differences between HMI and SO/PHI for an f-mode (c_t = 1.0) and α = 54°. The zero point of θ is chosen as the direction between the instruments. Black curves show results for ϕ_h = 0, red for ϕ_h = 45°. Solid curves are for SO/PHI and HMI observing from opposite azimuths, corresponding to the green diamond in Fig. 2. Dashed curves are for a case with 140° difference between the viewing azimuths, corresponding roughly to the points 30° (six points) away from the green diamond in Fig. 2. The dotted black curve is a cosine, for reference. The vertical dotted line shows the midpoint between the viewing directions, where the phase difference is zero. The azimuth is shown over more than 360° for clarity.