Coudé beam path
The Coudé beam path (often also referred to as Coudé for short ) is a beam path in astronomical telescopes whose focal point is fixed in the hour axis of the mount or in its own Coudé space . Two high-precision plane mirrors guide the beam path to the so-called Coudé focus. There is no rotation on the mount as with the Nasmyth telescope and no rolling, and the radiation sensors have a well fixed place. The word “coudé” means “bent over” in French.
The system was originally conceived for long telescopes in order not to expose the observer to the cold in a heated room. In the meantime, it has been implemented in various designs, especially on large reflector telescopes , and with large telescopes usually one of three selectable focus variants (primary focus of the main mirror , Cassegrain and Coudé focus). Until the introduction of heavy azimuthal mounts with resilient Nasmyth platforms and fiber optics, it was the only way to be able to operate immobile spectrographs that weighed tons , because they could then be accommodated in their own room.
The beam path initially corresponds to the Nasmyth system, but is then transferred to a fixed Coudé room with a series of mirrors. For this purpose, the beam path is either collimated or at least a very long focal length beam path is generated. However, the intermediate optics and mirrors required for this and for the transmission lead to noticeable light losses, so that Coudé beam paths are only used where other solutions such as light transmission through glass fibers are not possible. Astronomical interferometry is a field of application for Coudé beam paths.