Upper left panel: a close-up of the TrES-3 spectrum with the K I resonance double lines marked as two vertical blue lines, and three passbands used to generate the narrowband light curves in the panel on the right marked as light blue shaded areas. Lower left panel: a scatter map based on light curves generated using one pixel in the wavelength axis. Right: light curves generated by integrating over the whole spectral range shown in the left panels (W_a), integrating over the three marked passbands (W_b, i.e. excluding the core of the telluric O₂ absorption band), and integrated over the individual passbands (N₁–N₃).