Noise in PACS GOODS-S maps at 70, 100, 160 μm (top, middle, bottom panels), as a function of exposure time. The dashed line represent the trend σ_I ∝ t^-0.5 followed by instrumental noise; while the dotted line is σ_T obtained by summing σ_I and the confusion noise σ_c in quadrature. Squares mark the expected pure-instrumental noise at the exposure times considered, while circles denote the measured σ_T.