Sky and Telescope - July 2017 - 32
A Technology Past Its Limit
polishing to avoid surface distortions. Grinding the objective lenses also went slowly; only the photographic lens was
ﬁnished when the telescope was installed for the exhibition's
opening in April 1900.
At the Palais de l'Optique
The great telescope was the centerpiece of the exhibition's
"Palace of Optics," located right by the Eiffel Tower. It also
included a giant kaleidoscope, a hall of mirrors, and a hall
that could hold more than 3,000 people for lectures on
astronomy. A 60-meter horizontal tube was built in the Foucault Gallery, made of 24 cylinders of 2-mm sheet steel 1.5
meters in diameter. Resting on pillars towering 7 meters tall
and running the length of a dedicated exhibit space, the tube
weighed 21,000 kilograms. Stairs led up to viewing balconies
that ran the length of the tube on both sides. The ﬁnished
photographic objective lens was mounted at the north end. A
moveable section of tube at the south end let operators select
J U L Y 2 0 1 7 * SK Y & TELESCOPE
an eyepiece for visual use or a plate holder for photography.
The unﬁnished visual objective was displayed elsewhere.
The siderostat stood 10 meters tall at the north end of the
Foucault Gallery in the Paul Gautier Room, under sections
of roof and walls that could be slid to the north and south to
expose the sky. The base of the mirror's pivoting fork mount
ﬂoated in 60 liters of mercury to smooth its motion. A clock
drive kept the selected celestial object in the center of the
visual ﬁeld. However, objects rotated around the center as
the mirror turned to follow the sky, so photography required
another clock drive to rotate the plate holder.
As displayed, the whole telescope cost an estimated 1.4
million francs: roughly $300,000 at the time, or $8.2 million
today, and that did not include ﬁnal installation in an observatory. By comparison, the Carnegie Institution of Washington
would spend a similar amount, about $10 million in today's
dollars, building the immensely productive 100-inch Hooker
reﬂector in Mount Wilson Observatory. The 100-inch saw ﬁrst
© LÉON E T LÉ V Y / LÉON E T LÉ V Y / ROG ER VIOLLE T / THE IM AG E WOR KS
LIGHT PIPE Two hundred feet of steel pipe, weighing more than 10 tons, encircled the light beam on its way to focus. A cloth shroud or even just a
few light baffles might have done better; telescope makers have learned to keep as much mass as possible away from the light path to reduce distorting thermal effects. Observers thought that the city air was causing poor images, but the massive, unventilated metal tube could not have helped.