A memory includes a variable refresh control circuit for controlling the refresh rate of a memory array using a capacitor for data storage. In said steam power plant all components, including the fuel storage area, are situated at ground level and in the open air. The approximation and refinement may use a partitioning of function input variables to logic blocks to reduce the search space. En further elaboration, a lens is combined with a current limiting aperture, aligned such relative to a lens of said structure, that a virtual aperture effected by said current limiting aperture in said lens is situated in an optimum position with respect to minimizing aberrations total.