The Gregorian telescope employs a concave secondary mirror that reflects the image back through a hole in the primary mirror.

Click on the image to view it full size
The diagram also includes an embedded captioned figure:

The following in fact describes a "classic" Gregorian reflector with a concave parabolic mirror:
The Gregorian telescope consists of two concave mirrors; the primary mirror (a concave paraboloid) collects the light and brings it to a focus before the secondary mirror (a concave ellipsoid)
Modern large Gregorian telescopes (such as the Giant Magellan Telescope) are aplanatic, and instead use an ellipsoid primary as explained at Thus there is an intermediate abstract block GregorianReflector (which always has an ellipsoid secondary) and specific variants ClassicGregorianReflector and AplanaticGregorianReflector with different primary mirrors.

However, there is a very important property of the primary; it has a hole in it! From the point of view of the light flow model, light flows into the spatial region of the hole in the primary mirror then out of it, represented by the abstraction HoledMirror with additional ports for iHoleLight and oHoleLight. The ClassicGregorianReflector has a HoledParabolicMirror as primary and the AplanaticGregorianReflector has a HoledEllipsoidMirror as primary.

Up next
Snippets (quotes/extracts)
Visit also
Visit also (backlinks)
Related slides (includes other tutorials)
Related slides (backlinks, includes other tutorials)