Mask containing the spectral position and weights of 1828 absorption lines in the spectrum of the star WASP-121, within the spectral range of the HARPS spectrograph. Lines position are defined in the air in the stellar rest frame. Weights are set by the depth of the stellar lines. Photometry of WASP-121 with EulerCam at the 1.2m Euler telescope at La Silla, obtained during four independent transits of the ultra-hot Jupiter WASP-121b. The names of the files gives the date of the observation (year/month/day).