Among the many types of reflector telescope, the Gregorian represents the first, in order of time, to have been performed.
Designed by astronomer James Gregory in 1663, it is constituted by a concave primary mirror that reflects the light towards another secondary concave mirror, which in turn sends it to the eyepiece located beyond a hole made in the primary mirror.
It is a design very similar to the one that would later be adopted in the Cassegrain telescope, but with the difference that in the latter the secondary mirror is convex.
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