A parabolic mirror is not the same as a circular mirror. Fist, the focal point depends on the equation of the mirror.
If the equation is
y = a*x^2
where the origon is where the mirror crosses the surface, then the focal length is
f = 1/(4*a)
(Bauer and Westfall, pg. 1040)
Parabolic mirrors can be converging or diverging based on which side is reflecting:
the inside is converging, the outside is diverging.
Example:
(DuraVision Mirror Systems)
Home
References