Top: mean value of the sky noise estimates at 100 μm versus on-source integration time. Error bars are the rms standard deviation of the sky noise in the images taken with the same on-source observing time. The solid curve represents the noise behaviour assuming that the S/N ratio varies as the square root of the time, normalized by the mean value of the images with an on-source exposure time of 360 s. Bottom: the same for the 160 μm images.