Systems with apertures distributed longitudinally along a common axis with their edges falling on the surface of an imaginary ellipsoid of revolution have previously been shown to possess focusing capabilities. For better understanding of these capabilities, the width of the central lobe of the irradiance distribution in the principal focal plane and the energy enclosed in it are investigated. The width depends very greatly on the positions of the apertures; an increase in the number of apertures tends to decrease the width. The energy enclosed within the central lobe or, more precisely, within a circle of radius twice the half-width, increases with the number of apertures. For a particular system, the average energy spread over that circular area is independent of the positions of the apertures, and is independent of the half-width. Consequently, the total energy incident on that area is proportional to the square of the half-width.
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