Definition of narrowband channel #13b as a subchannel of channel #13. The gray shaded area encompasses the predicted K i resonance lines but avoids the high noise region caused by telluric oxygen absorption. Top panel: example spectrum of Ref2 within the limits of narrowband channel #13, showing the significant flux decrease in the telluric oxygen. Bottom panel: noise level at each wavelength as estimated from the after transit light curve standard deviation showing a strong increase of noise in the stronger telluric oxygen absorption band.