Raw images from HARPS-N taken before (HARPN.2017-04-09T10-06-46.946, left panel) and after (HARPN.2017-04-12T09-09-40.305, middle panel) a warm-up of the detector, and their ratio (right panel). The location of the core of the Ca II K and H lines in orders one, two, and three is highlighted by grey boxes. As we can see, the core of the Ca II K line in order one (K, bottom left) and the H line in order three (H) are strongly contaminated by ghosts. In addition, we also see that the contamination due to ghosts is significantly reduced with the warm-up process, which removes the humidity accumulated over time on the detector due to a small leak in its cryostat.