Sky and Telescope - July 2017 - 30
A Technology Past Its Limit
astronomers warned that no equatorial mount could handle
a telescope 60 meters long. Such an unwieldy tube would ﬂex
as it moved, and even if it could be kept rigid the observer
would have to move along with the eyepiece at nearly a foot
per minute to keep up with the turning of the Earth. Most of
all, it would require a 65-meter moving dome that would cost
several times more than the telescope itself.
To overcome these problems the group decided to build
a novel type of instrument, in which a large movable ﬂat
mirror would direct starlight into a stationary, horizontal
telescope. The mirroring device, called a single-mirror siderostat, had been invented in 1862 by the French physicist Léon
Foucault. A siderostat is a variation on the heliostat, a ﬂat
mirror that reﬂects sunlight in a constant direction as Earth
rotates. The siderostat could reﬂect light from objects in a
fairly large swath of sky and keep them ﬁ xed in the instrument's ﬁeld of view.
Foucault's single-mirror design minimizes light loss for
faint objects, but in practice it had been most successful for
solar astronomy. Gautier likely got the idea from Jules Janssen, the pioneering solar astronomer who discovered helium
in the Sun's spectrum; Janssen had built ﬁ xed horizontal
solar telescopes with siderostats at Meudon.
In the ﬁnal design, the telescope's key moving element was
a ﬂat, 2-meter silvered glass mirror 27 centimeters thick in a
fork-mounted siderostat. It directed light into the 1.25-meter,
f/48 objective and down a ﬁ xed tube.
With astrophotography becoming important, two interchangeable objectives were to be made: one achromatic in
the blue-violet wavelengths to which photographic emulsions
thick lenses even at f/19, seemed to be near a practical limit.
Deloncle had an interest in astronomy, and a desire to put
on a Paris fair that would outshine even the 1889 Paris exhibition highlighted by construction of the Eiffel Tower. His
dream was to build a telescope with such high magniﬁcation
that the Moon and planets would look almost within reach.
He found backers to form a company, Société d'Optique,
which sold stock to support building a new record-breaker
of a refractor - with a 60-meter (187-foot) focal length to
magnify images 6,000 times, at least in theory. To build it
they chose Paul Gautier, a mechanical engineer whose company had built the mechanical parts for many other French
telescopes, including Europe's largest refractor: the 83-centimeter (32-inch) Great Meudon Refractor at Paris Observatory's station in the suburb of Meudon.
A Bold and Different Vision
A magniﬁcation of 6,000× would make the moon look only
64 kilometers away, and the extremely long focus would
reduce troublesome spherical and chromatic aberration. But
J U L Y 2 0 1 7 * SK Y & TELESCOPE
BRINGING DOWN THE HEAVENS The 2-meter, flat siderostat mirror
rode in an exquisitely smooth equatorial mounting, which could keep
an object centered in the eyepiece more than 200 feet away for up for
CA RTOON: AUTHOR'S COLLECTION; PH OTO: © LÉON E T LÉ V Y / LÉON E T LÉ V Y / ROGER
VIOLLE T / THE IM AG E WOR KS
THE MASTERMIND "The Moon a meter away" reads the caption to a
satirical cartoon of François Deloncle and his telescope. The instrument
indeed served up impressive lunar views to countless paying visitors.
From the Encyclopédie du Siècle, published in 1900.