The Coude Focus
To study starlight in great detail, its spectrum must be made large enough to distinguish the finest features. But instruments for this kind of work must also be large--too large for the prime and cassegrain positions. Accordingly, the Shane telescope is equipped with a third focus, the coudé (pronounced koo-day), which directs the beam, no matter where the telescope is pointed, to a fixed place in the basement of the dome.

To accomplish this, a third mirror is added. As before, starlight fills the primary mirror and is reflected back toward the bottom of the telescope, converging toward a focus as it goes. As with the cassegrain focus, the light is intercepted by a second mirror near the top, but the coudé secondary causes the beam to converge even more slowly, lengthening the path it must travel before coming to a focus. After bouncing off the secondary, the now slowly converging beam travels downward and is intercepted by a flat mirror installed above the primary, about two thirds of the way down the telescope. This third mirror reflects the beam out of the telescope tube and down the hollow polar axle into the basement. Tilting the third mirror keeps the focused star image in one place as the telescope moves. The continually changing angle of the beam as it is reflected from the third mirror resembles the movement of a flexing arm, giving the coudé its name, from the French word for elbow.

Coudé instruments are free-standing and practically without restrictions on their size and weight. The 3-meter's coudé spectrograph occupies a partially subterranean room beneath the dome, slanting into the ground for about 30 feet along the angle described by the polar axle. Its floor is a broad flight of steeply descending steps and its low ceiling is hung with the optical components which comprise the spectrographs. Only the narrow beam of starlight from the telescope is allowed to enter the coudé room, where it traverses the darkness to form an exquisitely detailed spectrum on a digital detector.

However, the coudé's great spectral detail is bought at the price of efficiency. The additional reflections cost light, and the high magnification that results from the coudé configuration is unfavorable for spectroscopy. This, plus the intrinsic need for ample light for detailed examination, restricts the coudé to the study of relatively bright objects. Nevertheless, the coudé has provided Lick Observatory with some of its richest scientific returns.