Effect of the variation of the cluster effective temperature (T_eff, left) and an enhancement of the diffuse starlight and dust-scattered starlight radiation fields (right). Solid lines indicate the total IC spectrum. Dashed lines show only the cluster component on the left and the diffuse component on the right. Black lines show the best-fitting result using default parameters (Table 1), which is also displayed in Fig. 4 with the dotted blue line. The normalisation of all models is set equal to the default case.