Telescopes from the Ground Up

Newton’s Reflector

To eliminate chromatic aberration, Newton had to remove the glass. He replaced the primary glass lens with a concave mirror, called the primary mirror. The mirror would collect light and reflect it back through the telescope. The mirror focused light much like a glass lens, but because the light was reflected and didn’t pass through glass, it wouldn’t be split into colors.

Get to the root of it

At the time, no one knew how to make glass mirrors that would work in a telescope, so Newton made his mirror out of a metal alloy of copper and tin. He left the second glass lens, the eyepiece, alone. That lens would still cause chromatic aberration as the light passed through, but the lens was so small that the chromatic aberration was not noticeable.

In order to avoid blocking the light's path with his head as he looked through his telescope, Newton had to add another, smaller, flat mirror — the secondary mirror — inside the telescope. The secondary mirror reflected the gathered light out through the side of the telescope, into the eyepiece.

Compact but powerful

Newton’s first reflecting telescope, about six inches long, magnified objects almost 40 times — the same magnification as refractors that were 3 to 6 feet long. It contained a mirror that was about 6 inches in diameter.

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