Spherical Mirror vs Parabolic Mirror Spherical Aberration F

Spherical Mirror vs. Parabolic Mirror

Spherical Aberration • F • C Spherical Mirror • C F • Parabolic Mirror Only parallel rays close to the principal axis of a spherical mirror will converge at the focal point. Rays farther away will converge at a point closer to the mirror. The image formed by a large spherical mirror will be a disk, not a point. This is known as spherical aberration. Parabolic mirrors don’t have spherical aberration. They are used to focus rays from stars in a telescope. They can also be used in flashlights and headlights since a light source placed at their focal point will reflect light in parallel beams. However, perfectly parabolic mirrors are hard to make and slight errors could lead to spherical aberration. Continued…

Spherical vs. Parabolic Mirrors Parallel rays converge at the focal point of a spherical mirror only if they are close to the principal axis. The image formed in a large spherical mirror is a disk, not a point (spherical aberration). Parabolic mirrors have no spherical aberration. The mirror focuses all parallel rays at the focal point. That is why they are used in telescopes and light beams like flashlights and car headlights.

The Spherical mirror does not have a true optical axis, but if we drew a line from the center of the mirror, we would notice that light rays are not all focussed to one point, rather the light will focus at different points. This is called spherical aberration and its effect may be significant for smaller focal ratio's (< f/7), distracting for medium focal ratio's (f/8 - f/9) and negligible for longer focal ratios. A Parabolic mirror does not have this particular problem although off axis aberrations will be its bugbear (albeit these are correctable with specially designed lenses that slip into the focusser).

A spherical mirror, in principle, should work within the diffraction limit of a 4. 5" f/8 scope and thus be acceptable. However, in my experience, once one throws in other manufacturing errors / optical imprecision's, I have rarely found a spherical mirror primary Newtonian to work as well as its parabolic counterpart. Case in point, a C 4. 5 f/7. 9 Vixen with parabolic mirror usually outperforms the typical 4. 5" f/8 Newtonian with spherical mirror.