Upper panel: effective cavity width d(k) obtained for different absolute interference order numbers m₁. The reference curve (m₁ = 13 604, black) is shown in Fig. 2. Lower panel: difference in modeled FPI peak wavelength λ_m using m₁ = 13 704 with the reference curve (dashed line). Residuals of the spline fit to the cavity width d(k) obtained with m₁ = 13 704 are shown as purple dots.