Fig. 18. Convergence error of the standard (dashed-line) and of the nested (solid-line) multigrid methods as a function of the CPU time in units of the CPU time of one standard MG iteration (bottom scale) and of one MUGA iteration (top scale) in the finest grid. These are 2D multilevel calculations for our Ca II reference problem with =7.5 km, =31.25 km and for a horizontal periodicity P =750 km. The norm was used.