The New Heavens | Page 7

was available.
They were then shipped to California, and transported to the summit of
Mount Wilson over a road built for this purpose by the construction
division of the observatory, which also built the pier on which the
telescope stands, and erected the steel building and dome that cover it.
[Illustration: Fig. 12. Large irregular nebula and star cluster in
Sagittarius (Duncan).
Photographed with the 60-inch telescope.]

[Illustration: Fig. 13. Faint spiral nebula in the constellation of the
Hunting Dogs (Pease).
Photographed with the 60-inch telescope.]
The parts of the telescope which are moved by the driving-clock weigh
about 100 tons, and it was necessary to provide means of reducing the
great friction on the bearings of the polar axis. To accomplish this,
large hollow steel cylinders, floating in mercury held in cast-iron tanks,
were provided at the upper and lower ends of the polar axis. Almost the
entire weight of the instrument is thus floated in mercury, and in this
way the friction is so greatly reduced that the driving-clock moves the
instrument with perfect ease and smoothness.
The 100-inch mirror rests at the bottom of the telescope tube on a
special support system, so designed as to prevent any bending of the
glass under its own weight. Electric motors, forty in number, are
provided to move the telescope rapidly or slowly in right ascension
(east or west) and in declination (north or south), for focussing the
mirrors, and for many other purposes. They are also used for rotating
the dome, 100 feet in diameter, under which the telescope is mounted,
and for opening the shutter, 20 feet wide, through which the
observations are made.
A telescope of this kind can be used in several different ways. The
100-inch mirror has a focal length of about 42 feet, and in one of the
arrangements of the instrument, the photographic plate is mounted at
the centre of the telescope tube near its upper end, where it receives
directly the image formed by the large mirror. In another arrangement,
a silvered glass mirror, with plane surface, is supported near the upper
end of the tube at an angle of 45°, so as to form the image at the side of
the tube, where the photographic plate can be placed. In this case, the
observer stands on a platform, which is moved up and down by electric
motors in front of the opening in the dome through which the
observations are made.
[Illustration: Fig. 14. Spiral nebula in Andromeda, seen edge on
(Ritchey).

Photographed with the 60-inch telescope.]
Other arrangements of the telescope, for which auxiliary convex
mirrors carried near the upper end of the tube are required, permit the
image to be photographed at the side of the tube near its lower end,
either with or without a spectrograph; or with a very powerful
spectrograph mounted within a constant-temperature chamber south of
the telescope pier. In this last case, the light of a star is so reflected by
auxiliary mirrors that it passes down through a hole in the south end of
the polar axis and brings the star to a focus on the slit of the fixed
spectrograph.
ATMOSPHERIC LIMITATIONS
The huge dimensions of such a powerful engine of research as the
Hooker telescope are not in themselves a source of satisfaction to the
astronomer, for they involve a decided increase in the labor of
observation and entail very heavy expense, justifiable only in case
important results, beyond the reach of other instruments, can be secured.
The construction of a telescope of these dimensions was necessarily an
experiment, for it was by no means certain, after the optical and
mechanical difficulties had been overcome, that even the favorable
atmosphere of California would be sufficiently tranquil to permit
sharply defined celestial images to be obtained with so large an
aperture. It is therefore important to learn what the telescope will
actually accomplish under customary observing conditions.
Fortunately we are able to measure the performance of the instrument
with certainty. Close beside it on Mount Wilson stands the 60-inch
reflector, of similar type, erected in 1908. The two telescopes can thus
be rigorously compared under identical atmospheric conditions.
The large mirror of the 100-inch telescope has an area about 2.8 times
that of the 60-inch, and therefore receives nearly three times as much
light from a star. Under atmospheric conditions perfect enough to allow
all of this light to be concentrated in a point, it should be capable of
recording on a photographic plate, with a given exposure, stars about
one magnitude fainter than the faintest stars within reach of the 60-inch.

The increased focal length, permitting such objects as the moon to be
photographed on a larger scale, should also reveal smaller details of
structure and render possible higher accuracy of measurement. Finally,
the