Question

Why concave mirror of small aperture forms a sharper image?

Answer 1

A concave mirror of small aperture produces a sharper image because of the difference in the focal length. As the aperture of a lens (concave or convex) increases or decreases, its focal length also varies. The main reason for this is that in the case of large aperture, more than one focal length is formed from the same focus. This problem is called spherical aberration
Thus, a smaller aperture mirror will be free from spherical aberration, and produce a sharper image.