What is the difference between offset dish & prime focus / prime focul dish?

[RDosetareh]

What is the difference between a prime focal dish and an offset satellite dish?

The difference between the offset satellite dish and prime focal (prime focus) dish is the way the signal reflects from the surface and where it gets concentrated.
Prime Focus Dishes, the signal reflects from the satellite dish and concentrates towards the center, where the LNB is mounted to catch the signal.
Offset Dishes are designed with an offset angle. Usually set at 21-degrees. With this design, the signal reflects from the dish and concentrates towards the bottom of the satellite dish instead of the center of the satellite dish. See the pictures below:

Prime Focus Dish







Offset Dish