Deviation measures χ_C (upper panel) and χ_H (lower) versus the number of bins for the four stars (colours as in Fig. 1). Solid lines: deviation measures for the Q computed using 3, 4, , 11, 21, 31, 41, 51, and 101 τ-bins distributed equidistantly. Circles: deviations for Q computed with the Rosseland opacity. Pluses: the deviation of the optimal combinations of four τ-bins (see Sect. 5.1 and Table 4).