Question

Why refracting telescopes have spherical abberation?

Answer 1

Refracting telescopes are
Spherical aberration is due to the shape of the lens. A lens is nothing more than a piece of glass with each surface shaped like a portion of a sphere. Unfortunately, a sphere is not the best shape for concentrating light rays into a single point, but is the easiest shape to make when grinding and polishing glass. Spherical aberration causes images to be blurred.