Telescopes from the Ground Up

The birth of the reflecting telescope gives astronomers options

Newton was a busy man. He was the first to identify and begin to understand gravity. He discovered that white light is made up of colors, and he was among the first to formulate the mathematical discipline of calculus. And in 1668 in England, he created a small but powerful telescope that didn’t suffer from chromatic aberration.

Astronomers had struggled for years with chromatic aberration, the fringes of color that surrounded bright objects seen through a glass lens. As light passes through a lens, it breaks up into various colors because the glass bends the colors by different amounts. Newton’s solution was simple: He took the lens out of the telescope.

Newton replaced the primary lens with a polished, rounded, metal mirror. He experimented with different mixtures of metal and decided on one that was six parts copper to two parts tin. It was almost as bright as expensive, quick-to-corrode silver and would reflect a lot of light. The more light the mirror reflected, the better view the telescope would provide of the sky.

The light rays no longer passed through glass, so bright images were no longer surrounded by a colorful halo. Unfortunately, Newton couldn’t eliminate another common problem: spherical aberration, or the blurry view caused by the spherical shape of his primary mirror.

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Map showing the location of Cambridge, U.K.
Image of one of Newton's reflecting telescopes.Enlarge picture
Newton’s reflector
Year completed: 1668
Telescope type: Reflector
Light collector: Metal mirror
Mirror diameter: 6 inches
(15 cm)
Telescope length: 12 inches
(30 cm) long
Light observed: Visible
Discovery Highlights:
  • Newton made this telescope to study optics, not to observe the sky.