Schematic of the direct determination of from a single HARPS spectrum of the M1.5 V star HD 36395 (Karmn J05314−036). Its tabulated astrophysical parameters are T_eff = 3891 ± 51 K, log g = 4.64 ± 0.07, [Fe/H] = 0.23 ± 0.16 (Schweitzer et al. 2019) and an upper limit of v sin i = 2 km s⁻¹ (Reiners et al. 2018). Top panel: a PHOENIX synthetic spectrum with parameters T_eff = 3900 K, log g = 4.5, [Fe/H] = 0.0. Middle panel: the measured HARPS spectrum. The six bands used for rectification of the spectrum and subtraction of the photospheric flux (H, K, h₁, h₂, k₁, and k₂) are marked as shaded areas. Bottom panel: flux-calibrated spectrum (black line) after rectification of the measured spectrum with the gradient functions and, for comparison, the PHOENIX spectrum (gray line). The chromospheric fluxes (gray areas in bottom panel) are derived by subtracting the theoretical photospheric fluxes ℱ_H/K,phot (gray areas in upper panel) from the surface fluxes ℱ_H/K (gray areas in middle panel).