Optica Promota

work by Gregory

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July 1, 2026

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Optica Promota was a significant work published in 1663 by Scottish mathematician and astronomer James Gregory. This publication is primarily remembered for its description of the first practical reflecting telescope, now known as the Gregorian telescope.1

The Gregorian telescope design features a paraboloidal primary mirror with a hole in the center. Light is reflected by this mirror to a secondary, ellipsoidal mirror, which then directs the light through the perforation to the eyepiece. This design produces an erect image but has a limited field of view. Gregory's work analyzed the properties of lenses and mirrors, building upon Johannes Kepler's theories of the telescope.21

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James Gregory
…to London where he published Optica Promota (1663; “The Advance of Optics”). This work analyzed the refractive and reflective properties of lens and mirrors based on various conic sections and substantially developed Johannes Kepler’s theory of the telescope. In the epilogue, Gregory proposed a new telescope design with a secondary…
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…to London where he published Optica Promota (1663; “The Advance of Optics”). This work analyzed the refractive and reflective properties of lens and mirrors based on various conic sections and substantially developed Johannes Kepler’s theory of the telescope. In the epilogue, Gregory proposed a new telescope design with a secondary…
Read More