Modelled (T_F, ε_F) and observed (T_obs, ε_obs, +) apparent temperature and emissivity in the FP1 FOV during observation CIRS_222DI_DIONE001_PIE vs incidence (Inc) or emission (Em) angles. The bolometric Bond albedo A = 0.63 and the ice phase is amorphous. The topography of Dione GTM (q = 64) and the spatial sensitivity of FP1 are included. The emissivity of surface elements is assumed to be isotropic (Eq. (13)). Top: Modelled (T_F, ε_F) versus grain size, R, for thermal inertia Γ ~ 15 MKS assuming constant thermal conductivity k_E = 0.001 W m⁻¹ K⁻¹ and p = 0.5. Bottom: Modelled (T_F, ε_F) versus grain size, R, for an effective conductivity being both radiative and conductive. The contacts are tight and p = 0.7 so that the thermal inertia is in the range 2–23 MKS.