Different log R_HK,phot as a function of T_eff. The approximate spectral type is shown on the top axis. Top: PHOENIX stellar atmosphere models with different [Fe/H] and log(ɡ), where [Fe/H] is indicated by color and log(ɡ) indicated by symbol. The solid red line indicates a fifth-order polynomial fit. Bottom: calibrations of log R_HK,phot from other works where only the valid calibration region is plotted. Overplotted in red is the same fifth-order polynomial fit from the top panel.