Antenna, which whether transmits or receives correct signal or not is our main care and our direction, is an important part in any communication systems. Because of the characteristics of laser different from radio, the antenna system of point-to-point laser communication system must be designed strictly. Newton system, Green system and Cassegrain system are accepted commonly in optical antenna systems as field lens which include refraction system, inflection system and refraction-and-inflection system. In this paper, Cassegrain antenna is discussed. However, the potential problem of sheltering ratio in Cassegrain system leads to a loss of effective power of the optical system so called vignette phenomena. The other is related to optical aberration, because Cassegrain antenna sacrifices sight field for perfect image. We put forward some new solution and analyze theoretically how to reduce aberration based on the two considerations.