To be technically precise the reflector is spherical and not perfectly parabolic.
This in fact allows use of the reflector far away from 90deg without any astigmatic aberrations...just a diminishing gain the farther off axis.
A simple parabolic satellite dish can in fact be used several degrees off-center. This has been done for a long time with small ground-based antennae, i.e. systems for home reception of TV signals. They do suffer from a bit of astigmatic loss.
Okay. This is officially the poindexter thread.