Parabolic Mirror — Parallel rays coming into a parabolic mirror are focused at a point F. The vertex is V, and the axis of symmetry passes through V and F. For off-axis reflectors (with just the part of the paraboloid between the points P1 and P3), the receiver is still placed at the focus of the paraboloid, but it does not cast a shadow onto the reflector.
Photo Credit: Public Domain, https://commons.wikimedia.org/w/index.php?curid=3222214