(a) Synthesized PSD of the ℓ = 0, 2 modes of a star with ν_max = 275 μHz. The black modes are the true modes, whereas the blue modes are those observed because of the undersampling: they are the black modes mirrored around ν_Nyq. Each box of width Δν matches to one line of the echelle diagram shown in panel (b) along with its collapse in (c). In (b), the ridges due to the true modes are annotated in black, whereas those annotated in blue are due to the shadow modes. In (c) the peaks due to the mirrored modes are marked with a vertical dashed blue line and the peaks due to the true modes are marked with a vertical dotted black line.