Question

why is the aperture of spherical mirror taken small​

Answer 1

Spherical mirrors do not focus parallel light rays to a single focal point. The focus actually lies on a surface called a caustic. ... The point of using an aperture much smaller that the radius of the mirror is to restrict the incoming light to a region where rays do (almost) come to a single focal poin

Answer 2

it is taken small because if aperture is taken bigger then radius of curvature then the Principal focus may not come at the midpoint of principal axis