a) Comparison between different choices for the weights α of the penalty function in the reconstruction of the X-ray emissivity with constant smoothing scale L = 0.3h^-1Mpc. The expected result based on Eq. (10) is plotted for reference, whereas α = 0.0 corresponds to no regularisation. b) Comparison between different choices of the smoothing scale of the regularisation function in the reconstruction of the X-ray emissivity and α = 0.4. The expected result is plotted for reference.