present discussion that the sun 39 s image diameter has the same dimension d for the sun aligned condition depicted in fig2 the secondary mirror 12 casts a circular masking shadow upon primary mirror 11 so that solar radiation collected by mirror 11 must all be developed from incident parallel rays including and external to internal limiting rays a a the dimensions d and d the primary and secondary mirror radii and the relative axial positions of components 10 11 12 will also be understood to determine a primary to secondary ray b of first reflection attributable to each ray a and an inner secondary to cell ray b 39 of second reflection attributable to each ray b whereby ray b 39 impinges upon the remote transverse limit of the exposure surface of cell 10 in similar fashion an outer ray c reflected by the primary to the secondary mirror will be understood to determine an outer secondary to cell ray c 39 of second reflection attributable to each ray c whereby ray c 39 impinges upon the near transverse limit of the exposure surface of cell 10 it being noted that such limiting rays c 39 also establish a minimum value of the diameter h of the central aperture in mirror 11 and that the locus of all such rays c 39 is a geometrical cone of divergence angle u03b1 1 stated in other words the locus of all primary to secondary rays b is a geometrical cone of convergence angle u03b1 2 and any primary to secondary ray attributable to parallel light from the sun and striking mirror 11 at a point radially inside of the incidence of ray a on.