Visual representation of the problem of binning surface-brightness (S_X) bins into temperature-profile (kT) bins. Left: S_X profile of RXJ0856.1+3756 in the [0.3–2] keV band, extracted in 3$\stackrel{″}{.}$3 circular radial bins. Centre: Signal-to-noise ratio (S/N) in each bin of the S_X profile as a function of the bin index. Right: Innermost vs outermost S_X bin indexes, overplotted on the S/N obtained from placing the range of S_X bins indicated by the axes into one single kT bin. The green line delineates a 30σ S/N requirement. Two S_X example binning schemes are also shown (solid step curves). These correspond to a fixed binning of two input S_X bins (small fixed width steps), and a logarithmic binning of the S_X profile (continuously increasing step size).