Root mean square error on the transit spectrum of our fiducial hot Jupiter over the 1–2 μm window as a function of the final observed resolution. Dashed curves represent the error obtained with sampled cross-section tables at various sampling resolution (from top to bottom: R_sp = {1000, 3000, 10 000, 30 000}). The spectra are shown in Fig. 2. The black curve with dots represents the error using k-coefficients computed from high-resolution spectra directly at the final resolution. The gray curves show results when using binned down k-coefficients using natural (solid with stars) and non-integer (dotted) binning (see Sect. 3.4 for details).