  FAST, the largest single-dish radio telescope in the world, has a 500-meter
diameter main reflector and a 300-meter diameter illumination area. It has a
shape variable main reflector, which changes the shape of the illuminated area
in the main reflector into a paraboloid continuously. In this article, we
propose a quasi Cassegrain system to FAST. The detailed design results are
given in this paper. Such a quasi Cassegrain system only needs to add a
14.6-meter diameter secondary reflector, which is close to the size of the feed
cabin, the distance from the secondary reflector to the focus is only
5.08-meter, and it has excellent image quality. In this quasi Cassegrain system
the shape of the illuminated area in the main reflector continuously changes
into an optimized hyperboloid. Using this quasi Cassegrain system from
frequency 0.5 G to 8 G, the multi-beam system can include 7 to 217 feeds. If
this system is used in combination with PAF technology, more multi-beam feeds
can be used.
