Average SED for galaxy clusters. In black points we show the flux, as a function of frequency, found in the co-added maps obtained for a sample of 645 clusters. The error bars in black correspond to the dispersion estimated using bootstrap resampling (ΔF_b, Table 1). The red error bars have been estimated as the standard deviation of the flux integrated at random positions away from each cluster region (ΔF, Table 1). The black solid line shows the best-fit modified blackbody model (with β = 1.5). We note that the highest frequency point (60 μm from IRAS) is not used in the fit.