I'll add a little bit to Hobbes' excellent answer and detective work. I looked up a physical copy of the book linked there; Practical Conic Sections: The Geometrical Properties of Ellipses, Parabolas and Hyperbolas by J. W. Downs, Dover, NY, 1993 and found it a short but incredibly interesting and informative little book, if you like reading about conic sections.
From reading and enjoying the illustrations, I've learned that the hyperboloidal secondary does not have to be at all coaxial with the primary paraboloid. All sections of the paraboloid focus to a single point, so you can orient they hyperboloid any way you choose, so long as one of its foci coincide with the focus of the paraboloid.
Also, you can use a positive or negative hyperboloid, whichever suits your purpose. Hyperbolae come in pairs and are associated with two foci. Concave or convex, it will redirect the paraboloids rays converging to one of its focal points to a focus at the other point.
Finally, as discussed in the linked paper in Hobbes' answer, a motivation for building this unusual-looking reflector is that it prevents the feed horn from "seeing" the ground or atmosphere near the horizon where thermal noise is a serious problem. According to the abstract:
A new antenna type is described which combines the low noise temperature characteristics of the horn -ref lector antenna with the more attractive mechanical features associated with the paraboloidal reflector. Cassegrain optics used in an off-set feed configuration enables a virtual source to be formed without sub-reflector blockage. An extremely compact structure is realized with a concave hyperboloid which mirrors the actual feed located on the paraboloidal surface. Except for the aperture, the antenna is completely shielded. The design approach is outlined and measurements on an experimental model are presented. Ground noise contribution from minor lobes is about 2°K. (emphasis added)
From: A New Low Noise, High Gain Antenna S. R. Jones and K. S. Kelleher
Aero Geo Astro Corporation, Alexandria, Virginia, Reprinted from 1963 IEEE International Convention Record.
below: Figures 6.4 and 6.5 on pages 49-50 of Practical Conic Sections: The Geometrical Properties of Ellipses, Parabolas and Hyperbolas by J. W. Downs, Dover. 1993