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. Source Wikipedia
In the Gregorian design, the primary mirror creates a real 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. Source Wikipedia
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 ... Source Wikipedia
It is free of coma and spherical aberration at a nearly flat focal plane if the primary and secondary curvature are properly figured. Source Wikipedia
The Ritchey–Chrétien telescope ... is a specialized Cassegrain reflector which has two hyperbolic mirrors (instead of a parabolic primary). Source Wikipedia
The cassegrain telescope (sometimes called the "Classic Cassegrain") ... has a parabolic primary mirror, and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary. Source Wikipedia
A flat secondary mirror reflects the light to a focal plane at the side of the top of the telescope tube. Source Wikipedia
The Gregorian telescope ... employs a concave secondary mirror that reflects the image back through a hole in the primary mirror. This produces an upright image Source Wikipedia
