Two lens system: In analyzing the system we refer to Figure 1. However, Fig. 1 is only a schematic diagram of the optical arrangement. To minimize separation between optical elements one might use, for example, a diverging lens followed by a converging lens. A realization of this might be a reversed reflecting telescope of the Cassegrainian or Schwarzschild type in which the first reflector is a convex spherical (or hyperbolic) mirror which receives the incident solar radiation through an aperture in the larger spherical (or parabolic) mirror; alternatively one might use in place of the central opening axially off-set surfaces. Another realization of the schematic diagram of Fig. 1 might be a concave spherical (or parabolic) mirror followed by a Fresnel lens (zone plate). Thus by selecting appropriate focal lengths (d^ « fp « (d£ ^^2) an<^ sePara“ tion for the two lenses, the spot size on the earth can be made arbitrarily small (but is ultimately limited by diffraction effects). However the principal limitation of the system arises from the size of the required mirrors or lenses. Applying (2) and (3) again we note that Thus if the power density on the earth is specified to be 1/2 of that available in space, the size of the largest reflector becomes D$ « 234 km. This result, while not encouraging, does not rule out the passive reflector system since it may be possible to build and deploy even very large passive reflectors (Al foil or metal coated plastic) at reasonable cost. Likewise construction of very large Fresnel zone lenses consisting of alternate rings of
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