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Gregorian reflecting telescope (1663)
Diagram showing the light path in a Gregorian reflector telescope.

Long before the technology existed to make it, James Gregory envisioned a telescope with a parabolic primary mirror.

The telescope’s images would have been free of both chromatic and spherical aberration. By using a mirror, rather than a lens, Gregory eliminated chromatic aberration. The mirror's shape was parabolic, not spherical, eliminating spherical aberration.


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