Celestron Homepage

The NexStar 130SLT has a parabolic mirror.

Telescope mirrors are described by their figures in terms of the curvature of the optical surface. A spherical figure is one type of curvature which is like a section of a sphere’s surface, while a parabolic figure is another more complex curve. This curvature determines how the light is focused by the mirror.

A spherical mirror will reflect light coming from a distance and not bring all the rays to the same point or focus. Light striking outer parts of the mirror will focus closer than light striking the inner parts of the mirror. This effect and the blurriness it causes in an image is called spherical aberration.

A parabolic mirror will reflect light coming from a distance and bring all the rays to the same point or focus. It has no spherical aberration. However, a parabolic mirror  does have another effect called coma. It can be seen off to the side in images formed by the mirror.

The difference between the two curves and the resulting aberration also depends on focal ratio of the mirror. Larger f-ratio mirrors (bigger than f/8) will be nearly identical whether made as spherical or parabolic.