Top panel: from top to bottom, HD 209458 spectrum near Hα transition, our derived telluric spectrum, and the transmission spectrum. The selected pass-bands are ×1.5, ×3, ×4.5 the stellar line width, and the 16 Å pass-band used in Jensen et al. (2012). Bottom panel: transmitted signal measured in each used spectral window, including the ×21 (16 Å). This panel shows a possible detection of absorption excess due Hα in the narrower pass-band.