Typical power spectra of several components contained in observed Herschel images. Red circles show the power spectrum of the SPIRE 250 μm image of the Polaris cirrus cloud taken as part of the Herschel Gould Belt survey; no attempt was made here to correct the raw power spectrum for deviations of the SPIRE 250 μm beam from a Gaussian shape (cf. Miville-Deschênes et al. 2010). This cirrus background is well described by a power law P(q)    ∝    q^-3 in the range of spatial frequencies 0.02    <    q    <    2 arcmin^-1. Blue squares show the estimated power spectrum of the instrumental noise in a typical Herschel image at 250 μm, which is flat over more than two orders of magnitude in q. The solid curve shows the power spectrum (scaled to an arbitrary level of power) of the intensity distribution of a Gaussian source with a size of 1 arcmin (FWHM). The vertical dashed line marks the spatial frequency of 0.46 arcmin^-1, at which the contrast of such a source is maximum over the background fluctuations (assumed to follow P    ∝    q^-3).