  The effect of aberrations on the aperture efficiency has not been discussed
analytically, though aberrations determine the performance of a wide
field-of-view system. Expansion of a wavefront error and a feed pattern into a
series of the Zernike polynomials enables us to calculate the aperture
efficiency. We explicitly show the aperture efficiency affected by the Seidel
aberrations and derive the conditions for reducing the effects of the spherical
aberration and coma. In particular, the condition for coma can reduce a
pointing error. We performed Physical Optics simulations and found that, if the
Strehl ratio is higher than 0.8, the derived expression provides the aperture
efficiencies with a precision of < 2%.
