Augmented mask of order 130. The intensity of the stacked spectrum (high S/N) after correction of telluric absorption is shown in black (left scale). The atmospheric transmission computed from TAPAS (green, right scale) displays the γ band of O₂. The original binary mask BM1 contains only three lines, whose wavelengths are indicated at the bottom by red vertical bars. These lines avoid the O₂-contaminated region. A new Binary Mask BM2 containing 36 lines is made from the observed stacked spectrum with stellar lines whose depths are ≥0.2. The wavelength assigned to the stacked spectrum is that of exposure number 100, in the middle of the series of exposures. The three red lines of BM1 are seen shifted to the left from their blue-line counterpart detected in the stacked spectrum.