Because the mirror focuses the light back along its central axis, the secondary mirror must be on that axis, causing it to block the central portion of the light path.
(the mirror; focuses; the light; back along its central axis) (the secondary mirror; must be; on that axis) (the secondary mirror; causing; it to block the central portion of the light path) Context(the secondary mirror causing,List([66, 116))):(it; to block; the central portion of the light path)
the secondary mirror causing,List([66, 116))
the mirror the secondary mirror the secondary mirror it
focuses must be causing to block
the light back along its central axis
on that axis
it to block the central portion of the light path
the central portion of the light path