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Parabolic shape eliminates spherical aberration in telescope mirrors
Diagram demonstrating the different focal points created by a spherically shaped mirror versus a parabolic shaped mirror.

Spherically shaped lenses and mirrors share a problem: their shape. Parallel light rays that bounce off the central region of a spherical mirror focus farther away than light rays that bounce off the edges. This results in many focal points, which produce a blurry image. To get a clear image, all rays need to focus at the same point.

Changing the shape of a mirror from spherical to parabolic solves the problem. All light rays focus at the same point and the resulting image is sharp and clear.

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