According to the laws of optics and analytical geometry,
for this type of reflector all reflected rays will be parallel to the axis of the paraboloid which gives us ideally one single reflected ray parallel to the main axis with no sidelobes.
The field leaves this feed horn with a spherical wavefront. As each part of the wavefront reaches the reflecting surface, it is shifted 180 degrees in phase and sent outward at angles that cause all parts of the field to travel in parallel paths.