Design
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) where it is reflected back through a hole in the centre of the primary, and thence out the bottom end of the instrument where it can be viewed with the aid of the eyepiece.
The Gregorian design solved the problem of viewing the image in a reflector by allowing the observer to stand behind the primary mirror. This design of telescope renders an upright image, making it useful for terrestrial observations. It also works as a telephoto in that the tube is much shorter than the system's actual focal length.
The design was largely superseded by the Cassegrain telescope. It is still used for some spotting scopes because this design creates an erect image without the need for prisms. The Steward Observatory Mirror Lab has been making mirrors for large Gregorian telescopes at least since 1985.
In the Gregorian design, the primary mirror creates an actual image before the secondary mirror. This allows for a field stop to be placed at this location, so that the light from outside the field of view does not reach the secondary mirror. This is a major advantage for solar telescopes, where a field stop (Gregorian stop) can reduce the amount of heat reaching the secondary mirror and subsequent optical components. The Solar Optical Telescope on the Hinode satellite is one example of this design.
For amateur telescope makers the Gregorian can be less difficult to fabricate than a Cassegrain because the concave secondary is Foucault testable like the primary, which is not the case with the Cassegrain's convex secondary.
Read more about this topic: Gregorian Telescope
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