Comparison of the spectropolarimetric observations of SN 1998S at 5 d after explosion with the counterparts obtained with LONG_POL assuming a 2D, prolate ejecta (ρ_pole/ρ_eq = 5) based on model r1w6b at 5 d. The observer’s inclination relative to the axis of symmetry is 90 deg. Four ISP choices are shown, corresponding to ISP[1,2,3,4] indicated in Fig. 3. Note: “ISP2” represents the observed data, with no ISP removed (the residuals for each case are shown in Fig. C.1). Observations have been corrected for a recession velocity of 840 km s⁻¹ and a reddening E(B − V) of 0.11 mag. Model spectra have been smoothed and rebinned to 6 Å to match the resolution of the observations. See Sect. 4 for a discussion.