Transmission spectra of HD 209458 as a function of light frequency, highlighting different wiggle patterns at the start (exposure 14, top panel) and end (exposure 79, bottom panel) of epoch 2. Top (resp. bottom) subpanels show data before (resp. after) wiggle correction. The wiggle model, overplotted as a dashed red line, is a beat pattern between a dominant (blue) and weaker (orange) component. Right panels show periodograms of the data (horizontal dashed lines are false-alarm probability levels at 1, 5, and 10%), highlighting the removal of these components.