High-resolution transmission spectrum of WASP-21b at the Na doublet and Hα lines. The black circles with error bars show the 10×binned transmissionspectrum, while the gray line shows the 0.01 Å version. The red line presents the best-fit Gaussian function to the Na doublet and Hα lines. The green lines and shaded areas refer to the median and 1σ confidence region of the Gaussian fit. Top panel: transmission spectrum, bottom panel: the one after subtracting the best-fit model.