Abstract: A secondary optical system to increase efficiency in collecting solar radiation concentrated by primary mirrors in “beam-down” plant configurations is described. Such secondary optical system is formed at least by two reflectors—preferably in form of plane mirrors arranged at different heights—oriented to associate to each of them a sub-section of a primary field of heliostats. The secondary reflectors—each one associated to a different primary focus F1,i (i=1, . . . , n) corresponding to a sub-set of primary concentrators—allow concentration of solar rays towards a single secondary focus F2.