Example of on-sky raw thermal background measurements obtained in the N’ band with the telescope pointing at an empty region of the sky and covering approximately 15 degrees of elevation change during the whole duration of the sequence. The left panel shows the flux integrated over a photometric aperture of 8 pixels in radius while the right panel shows the corresponding distribution. The complete figure and a more detailed analysis of the performances of the nulling data-reduction performances for the HOSTS survey can be found in Defrère et al. (2016).