The diagram at the left shows how the approximated parabolic mirror is formed out of two spherical sections. The Grey area is the glass, the Red curve is the Parabolic Curve being approximated, the Black Curve is a segment of the Larger Outer Sphere, and the Blue Circle is the Inner sphere. The color bands show the zones controled by different areas of the mirror's surface.
In a True Parabolic Mirror, all incomming parallel light rays striking the mirror will pass through a single common focal point along the mirror's axis of symmetry.  This is not true for Spherical Mirrors. All spherical mirrors require the assistance of a Corrector Plate, which bends the incomming parallel light rays so that when they strike the mirror's surface they will be reflected through a single common focal point along the mirror's axis of symmetry.
Without a Corrector Plate, a spherical mirror will bring many incomming parallel light rays to a common focal point along the mirror's axis of symmetry. However, it will reflect most of those incomming light rays inwards to cross the axis of symmetry at numerous points all along that axis.   Reset image to Glass Blank.