Telescopes from the Ground Up

Double vision

Kepler was unable to solve the issue of chromatic aberration, circles of color that would appear around bright objects due to the way lenses bend light. Lenses bend different colors of light by different amounts, creating a different focal point for each color.

But he did discover a way to get rid of the blurry images produced by Galileo’s telescope. Kepler found that this problem was caused by the spherical shape of the primary lens. Spherical lenses cause light rays to bend so that they never meet at one point. The blurriness this caused became known as spherical aberration.

By studying the lenses of the human eye, Kepler learned that a more complicated shape than a sphere would be needed to make a lens focus perfectly.

Astronomers did not yet have the technology to grind lenses of different shapes, but they could make spherical lenses with a more gradual curve, which caused the light to bend less. This made the light rays come closer to meeting at a single point. Unfortunately, the light rays met much farther away from the eyepiece.

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