Gregorian telescope

The Gregory telescope, even Gregorian telescope is a reflecting telescope was invented by James Gregory in 1670 and first built by Robert Hooke in 1674.

In contrast to the Cassegrain telescope, a concave secondary mirror beyond the primary focus is used, which casts the image through a central hole in the primary mirror to the secondary focus. The design was used for ease of manufacture concave secondary mirror until the 19th century, but was overshadowed by the more successful because of shorter construction Cassegrain. Since the Gregory telescope delivers an upright image, small spotting scopes are manufactured in this design in order to save the inverting prisms today. A modified structure according to Gregory, with additional mirrors for further folding the beam path, is used in some telescopes to observe the Sun in the optical and millimeter wave and submillimeter wave, for example, when Viper telescope in Antarctica or the Green Bank Observatory. Modern optical instruments of this type are the two 6.5m Magellan telescopes and the 1.5 m GREGOR telescope on Tenerife. One reason for the choice of the Gregory - design is the ability to access the primary focus without removing the secondary mirror. An example of this is the Effelsberg radio telescope.

Disadvantages compared to a comparable Cassegrain telescope are the larger secondary mirror, which thereby may prompt stronger vignetting and the longer tube, which is not only wind prone, resulting in a more stable mount requires, but also for a larger dome.