Telescopes from the Ground Up

Astronomers solve two major telescope flaws

Refracting telescopes, with their long, difficult-to-handle design, were quickly being outpaced by the more practical reflecting telescopes. They might have dropped out of the race altogether — but discoveries about light and the way glass lenses work led to innovations that solved two nagging problems.

By the mid-1700s, astronomers understood that glass would separate, or disperse, white light into a rainbow of colors. In refracting telescopes, this effect created a problem called chromatic aberration — circles of color surrounding bright objects. Chromatic aberration occurs because lenses bend different colors of light by different amounts. Red light bends the least and violet light bends the most, creating a focal point for each color.

Around the bend

Astronomers also realized that some kinds of glass bend light more than others. In 1729, Chester Moor Hall of England, a hobbyist experimenting with lenses, realized he could use this effect to solve the problem of chromatic aberration. He combined a concave lens of dense, clear “flint glass,” the type used to make cut-glass decorations, with a convex lens of “crown glass,” the type used in windows.

Image above: Courtesy Yerkes Observatory
Click here to see all avaliable eras.