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