Summary of the observations of WASP-121 b. The top panel is the air mass as a function of phase for all observations. The bottom panel shows the S/N per pixel throughout the observations. The grey shaded area centred around phase 0.5 represents the phases where WASP-121 b is eclipsed by its host star. Of the six observations, three of them were obtained during the pre-eclipse phases (phases <0.5), while the other three are of the post-eclipse phases (phases >0.5).