Rayleigh-Jeans temperature of Saturn’s ring as a function of frequency, as estimated by WMAP, ACT, and Planck HFI, plotted together with a power-law model fit to all measurements. The best-fit model (solid line) suggests a spectral index of β_ring = 2.30 ± 0.03 compared to a spectral index of β = 2 for a perfect RJ source. The grey region is there to remind the reader that the sub-mm bands are calibrated on models of the brightness of Uranus and Neptune (see Sect. 2.1.2).