Simulated spectra of a CSP at 13 Gyr from the onset of the SFH (exponentially delayed with τ = 0.3 Gyr), with solar metallicity and S/N 2, 5, 10, 20 and 30, increasing from bottom to top. For each S/N, the blue curve is the starting BC03 model used for the simulation, while the red curve is the simulated spectrum. The lower panel shows the behaviour of the S/N as a function of wavelength for a simulated spectra with S/N = 30 in the 6500–7000 Å window.