Sky and Telescope - July 2017 - 29

LEF T: ENG R AVING BY POYE T; ROG ER VIOLLE T / THE IM AG E WOR KS
A BOVE: WIKIMED IA COMMONS

new generation of gigantic telescopes should reach ﬁrst
light in the 2020s: the 24-meter Giant Magellan Telescope (GMT), the Thirty Meter Telescope (TMT), and
the 39-meter European Extremely Large Telescope (E-ELT).
Only then, and after ﬁnal shakedown and tweaking, will
their actual capabilities be known. Their designers and
engineers are skating at the edge of 21st-century technology.
And in any given era, including recently, the world's most
ambitious telescopes have had only a mixed record of reaching their goals (S&T: Jan. 2015, p. 60).
So perhaps it's time to remember the largest refractor
that was ever built. If you think this was the Yerkes 40-inch,
commissioned in 1895 and famous ever since, you're wrong.
That was the largest productive refractor ever built, and it's
still in service (for education and public outreach). The
actual largest refractor was the Paris 49-inch, considered a
triumph of optics and engineering ... for a while. Its story
should be better known.
The 40-inch and 49-inch refractors each came into
public view with a grand splash at a world's fair. In 1893
the Warner & Swasey engineering ﬁrm displayed the tube
and mount for the Yerkes 40-inch at the Columbian Exposition in Chicago. Not to be outdone, at the 1900 Exposition
Universelle in Paris a French ﬁrm debuted an even bigger
working refractor, with a 1.25-meter (49¼-inch) objective
and a gigantic, 60-meter (200-foot) tube. But while the Yerkes telescope became a workhorse, the Great Paris Telescope
made only a few noteworthy observations before its owners
went bankrupt, no astronomers or institution would buy it,
and it was sold off as scrap metal.
What went wrong?
Refractors were already reaching their practical limits.
The lenses of the 40-inch were so thick that, it was argued, a
larger objective lens would never be worth making, because
the increasing thickness of the glass would absorb as much
light as the larger aperture would gain. Thinner lenses would
require very shallow curves on their surfaces, making the
focal length so long that the tube could never be mounted.
An ever-longer f/ratio would also be needed to keep chromatic aberration under control.
But at the same time, professional astronomers were wary
of gambling on giant reﬂectors, especially following the
disappointment of the Great Melbourne Reﬂector in Australia. Built in 1868 with a 48-inch speculum-metal mirror, it
became widely regarded as an expensive failure.
The Great Paris Telescope was an oddity from the start.
It was the brainchild not of an astronomer but a member
of the French Parliament, François Deloncle. He wanted
to show off French technology and science at the 1900
Paris fair in the grandest manner possible. After talking with Maurice Loewy, director of Paris Observatory,
Deloncle decided a record-breaking refractor would be just
the thing. (The idea of a 120-inch reﬂector was considered
and rejected.) His quest led to construction of a unique and
innovative instrument that initially seemed to work well.

New electric lights
blazed across the 1900
Paris Exhibition at night,
especially around the
Palace of Electricity.

It was the largest refractor ever
built, with a 49-inch objective
lens. It worked. And then it was
sold as scrap.
A Time of Telescopic Transition
Reﬂectors were the ﬁrst giant telescopes, Melbourne notwithstanding. As early as 1789 William Herschel had built a
48-inch reﬂector with a 40-foot focal length for high magniﬁcation. But it proved cumbersome to use, and its speculum-metal mirror required frequent repolishing and hence
reﬁguring. In 1845 William Parsons, the third Earl of Rosse,
built a 72-inch speculum-metal reﬂector with a 54-foot tube.
His "Leviathan of Parsonstown" gave great views when the
weather was good, but that was rare on his Irish estate, and
the massive tube was hard to control (it required several
assistants pulling ropes while the observer gave orders). And
it could only see near the meridian. By 1890 his son had
abandoned it for a smaller telescope that was easier to use.
Undeterred by these experiences, the wealthy English
amateur Andrew A. Common built a 60-inch silvered-glass
reﬂector in 1890, but he also found its use very cumbersome.
Refractors, however, were in their heyday. Lick Observatory's 36-inch refractor in California set a size record in
1888, and the Yerkes 40-inch in Wisconsin soon surpassed
it. Astronomers liked the image quality of refractors; their
optics were less difﬁcult to ﬁgure well, while early reﬂector
surfaces tarnished quickly and may have scattered more light.
But the Yerkes refractor, at 60 feet (18 meters) long and with
s k y a n d t e l e s c o p e .c o m

* J U LY 2 017

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Sky and Telescope - July 2017

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