A radio telescope has been used and has worked as one of the most important tools to observe various objects, e.g., molecular clouds, galaxies, and the universe, for several tens of years. Techniques accompanying a radio telescope, including VLBI, are quite mature. These techniques allow us to observe with high sensitivity, high spatial resolution, and high frequency resolution. On the other hand, there remains a problem in designing an optical system for a radio telescope. We have not obtained a systematic method to make the view of a radio telescope as large as possible. In this talk, I will show you an approach using geometrical optics and quasi-optics. It leads us to the analytical expression of aperture efficiency affected by aberrations. It will be also shown that the expression tells the conditions to compensate aberrations different from those known in geometrical optics and has potential to be applied to various fields, e.g., an optical telescope, laser optics, and so on.