Same as in Fig. 2 but with manually added white-Gaussian noise at a signal-to-noise level in the r-band of 5 in a), and of 1 in b). The red lines indicate the empirically-determined continua in each case. Many of the prominent lines are easily visible by eye at the higher S/N of 5, whereas at the lower S/N of 1 most of the lines are obscured, with only H_α being sufficiently prominent so as to be detectable. The continuum estimate is good at the S/N of 5, and comparatively poor, but of the correct order of magnitude, at the lower S/N due to the dominating influence of noise. As an indication of line-S/N, we quote the values for the S/N on H_α for these particular spectra as 8.9 and 1.7 respectively for a) and b).