In telecommunications and radar, a Cassegrain antenna is a parabolic antenna in which the feed antenna is mounted at or behind the surface of the concave main parabolic reflector dish and is aimed at a smaller convex secondary reflector suspended in front of the primary reflector. The beam of radio waves from the feed illuminates the secondary reflector, which reflects it back to the main reflector dish, which reflects it forward again to form the desired beam. The Cassegrain design is widely used in parabolic antennas, particularly in large antennas such as those in satellite ground stations, radio telescopes, and communication satellites. Geometry The primary reflector is a paraboloid, while the shape of the convex secondary reflector is a hyperboloid. The geometrical condition for radiating a collimated, plane wave beam is that the feed antenna is located at the far focus of the hyperboloid, while the focus of the primary reflector coincides with the near focus of the hyperboloid. Usually the secondary reflector and the feed antenna are located on the central axis of the dish. However, in offset Cassegrain configurations, the primary dish reflector is asymmetric, and its focus, and the secondary reflector, are located to one side of the dish, so that the secondary reflector does not partially obstruct the beam. Advantages This design is an alternative to the most common parabolic antenna design, called "front feed" or "prime focus", in which the feed antenna itself is mounted suspended in front of the dish at the focus, pointed back toward the dish. The Cassegrain is a more complex design, but in certain applications it has advantages over front feed that can justify its increased complexity: The feed antennas and associated waveguides and "front end" electronics can be located on or behind the dish, rather than suspended in front where they block part of the outgoing beam. Therefore, this design is used for antennas with bulky or complicated feeds, such as satellite communication ground antennas, radio telescopes, and the antennas on some communication satellites. Another advantage, important in satellite ground antennas and radio telescopes, is that because the feed antenna is directed forward, rather than backward toward the dish as in a front-fed antenna, the spillover sidelobes caused by portions of the beam that miss the secondary reflector are directed upwards toward the cold sky rather than downwards towards the warm earth. In receiving antennas this reduces reception of