where formula_58 is the annular aperture obscuration ratio, or the ratio of the diameter of the obscuring disk and the diameter of the aperture (beam). formula_59, and x is defined as above: formula_60 where formula_11 is the radial distance in the focal plane from the optical axis, formula_13 is the wavelength and formula_63 is the f-number of the system. The fractional encircled energy (the fraction of the total energy contained within a circle of radius formula_11 centered at the optical axis in the focal plane) is then given by: