Airmass plots for KELT-9 b observations. Each data point represents an individual exposure frame. In-transit, ingress/egress, and out-of-transit exposures are indicated with red, orange, and blue. The civil, nautical, and astronomical twilight phases are indicated by the blue shaded regions. All transits were observed at low airmass. Parts of the transits observed with FOCES occurred during astronomical twilight given the short duration of the nights in summer.