Telescopes from the Ground Up

The reflecting telescope gets an improved mirror

Reflecting telescopes had contained spherical metal mirrors since Isaac Newton’s time, but not out of choice. Spherical mirrors, like spherical lenses, produced blurry images of the sky. To create a clear, sharp image, the mirrors would have to be a different shape — a more sharply rounded shape called a “paraboloid.”

Astronomers had known about the possibilities of parabolic mirrors since 1663, when James Gregory, a mathematician in England, envisioned a telescope that would bounce light between two mirrors, one with a hole in it to allow light to reach the eyepiece.

Ahead of his time

Gregory wanted his primary mirror to be parabolic, to get rid of the blurry image caused by spherical aberration, but no one could grind surfaces to that shape during that time period. His design was right, though, and today we call such telescopes “Gregorian reflectors.”

The first person to actually create a parabolic mirror was another English mathematician, John Hadley. In 1721, he built a Gregorian reflector whose mirror had very little spherical aberration.

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Early Reflectors
Map of East Barnet, UK, where the Hadley's reflector was located.
Illustration of Hadley's reflector.Enlarge picture
Hadley’s Reflector
Year completed: 1721
Telescope type: Reflector
Light collector: Metal mirror
Mirror diameter: 6 inches
(15 cm)
Light observed: Visible