Why are mirrors used in search light are parabolic not concave.

In search lights , we need an intense parallel beam of light . If a source is placed at the focus of a concave spherical mirror , only paraxial rays are rendered parallel . Due to large aperture of mirror , marginal rays give a divergent beam .

But in case of parabolic mirror , when source is at the focus , beam of light produced over the entire cross-section of the mirror is a parallel beam .