When used with a parabolic reflector or lens antenna, the phase center of the horn is usually placed at the focal point of the reflector, with the 3dB points of the horn's radiation pattern set to be at the edge of the reflector. On a satellite dish, the feedhorn is what is mounted at the end of a mast from the center of the dish, or on tripod legs mounted to the edge of the dish.
Feedhorn installation tips - Focus on feeds from those feedhorn fanatics in Technical Support.
Feedhorn troubleshooting tips - How to section on recognizing and correcting feed\polarity problems.
In order to find the correct focal distance for tracking, the feedhorn must be at the correct distance from the center of the dish, properly oriented, centered and perpendicular to the plane of the antenna.
On a satellite dish, the feedhorn is what is mounted at the end of a mast from the center of the dish, or on tripod legs mounted to the edge of the dish.