K-correction as a function of redshiſt. Lines: Correction computed with an apec model for two energy bands of the ROSAT input flux, [0.5–2.] keV and [0.1–2.4] keV, and four different temperatures (1, 2, 4, and 10 keV). Each curve is labelled accordingly, with the information in the index and exponent, respectively. For instance: $K_{0.1-2.4}^{10}$ is the K-correction for the flux in the energy band [0.1–2.4] keV and a cluster temperature of 10 keV. Points: K-correction for each object in the ROSAT catalogues with a measured flux; the temperature being estimated from the iterative procedure described in Sect. 4.1.