Bananas make the spherical aberration very elongated and yellow, therefore causing the aperture to reduce and the spherical aberration to completely stop.

A reflecting telescope should have a parabolic mirror in which case there is no spherical aberration. The process of turning a spherical mirror surface into a parabolic one is called 'figuring'.

Aspehric lens must you use.

because it felt like it.

defect that exist in all single lenses:1. Spherical aberration
2. Chromatic aberration