With a reflector that large the movement of the feed where there is gain to point off a bit must be only a few tenths of a degree, depending on frequency.
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.