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coudé telescopeAny telescope in which the light emerges along the polar axis so that its direction remains fixed while the object observed goes through its normal diurnal motion. This arrangement is essential when the light is to be subjected to analysis by bulky and delicate instruments, such as the spectrograph, which can then be mounted in a fixed position at the coudé focus.
There are two main ways in which the fixed coudé focus can be secured. In one alternative, involving an equatorially mounted telescope, one or more extra mirrors can be placed so as to reflect or ‘bend’ the emerging light along the polar axis – the French word coudé means ‘bent’ or ‘angled’ (see illustration). This is the practice in large reflecting telescopes where the coudé focus is provided in addition to the prime and Cassegrain foci. It is also the basis for the amateurs' Springfield mount, in which the diagonal of a Newtonian telescope sends the light along the declination axis to an additional diagonal, which reflects it up the polar axis to the observer's eye.
In the other alternative, the whole telescope is mounted in a fixed position along the polar axis; a large moveable plane mirror is driven by a mechanism that allows it to reflect the observed object into the telescope for as long as observations may continue. This is the basis of many solar telescopes.