The Great Paris Exhibition Telescope of 1900, with an

objective lens In optical engineering, the objective is the optical element that gathers light from the object being observed and Focus (optics), focuses the ray (optics), light rays to produce a real image. Objectives can be a single Lens (optics), lens or mirr ...

of in diameter, was the largest

refracting telescope A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens (optics), lens as its objective (optics), objective to form an image (also referred to a dioptrics, dioptric telescope). The refracting telescope d ...

ever constructed. It was built as the centerpiece of the Paris Universal Exhibition of 1900. Its construction was instigated in 1892 by François Deloncle (1856–1922), a member of the French

Chambre des Députés Chamber of Deputies (french: Chambre des députés) was a parliamentary body in France in the nineteenth and twentieth centuries: * 1814–1848 during the Bourbon Restoration and the July Monarchy, the Chamber of Deputies was the lower house ...

. Since it was built for exhibit purposes within a large metropolis, and its design made it difficult to aim at astronomical objects, it was not suited for scientific use. When the year-long exposition was over, its builders were unable to sell it. It was ultimately broken up for scrap; the lenses are still stored away at the

Paris Observatory The Paris Observatory (french: Observatoire de Paris ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centers in the world. Its histor ...

Design

The telescope had two interchangeable objective lenses (for visual and photographic use respectively) in diameter, with a

focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative foca ...

of . Due to its extremely large size, the telescope was mounted in a fixed horizontal position. Light from astronomical objects was redirected into the optical tube assembly via a Foucault

siderostat A heliostat (from ''helios'', the Greek word for ''sun'', and ''stat'', as in stationary) is a device that includes a mirror, usually a plane mirror, which turns so as to keep reflecting sunlight toward a predetermined target, compensating f ...

, a movable plane mirror in diameter, mounted in a large cast-iron frame at the objective lens end of the telescope. The horizontal steel tube was long. The telescope's eyepiece/plate end could be shifted five feet on rails for focusing. With the lowest power of 500×, the field of view was 3 arc minutes.

Construction of the telescope

The mirror for the siderostat was ground mechanically by the Gautier Company (headed by Paul Gautier, 1842–1909) and took nine months to finish. The blank for the mirror was cast by Georges Despret, director of the Jeumont glassworks in northern France. The lens blanks were cast by Édouard Mantois (1848–1900) and ground by Gautier. By the time the Paris Exhibition opened only the object lens for photographic observation was ready. The visual object lens, unfinished, was put on display nearby.

Erection of the telescope

The telescope was erected in the Palais de l’Optique on the

Champ de Mars The Champ de Mars (; en, Field of Mars) is a large public greenspace in Paris, France, located in the seventh ''arrondissement'', between the Eiffel Tower to the northwest and the École Militaire to the southeast. The park is named after the ...

, near the

Eiffel Tower The Eiffel Tower ( ; french: links=yes, tour Eiffel ) is a wrought-iron lattice tower on the Champ de Mars in Paris, France. It is named after the engineer Gustave Eiffel, whose company designed and built the tower. Locally nicknamed "'' ...

. The tube, oriented north-south, was made up of 24 cylinders in diameter and rested on 7 concrete and steel pillars; its axis was above the floor. The room at the end which housed

with the mirror had a movable dome to allow direct access to the sky.

Scientific observations

A few scientific observations were made using the giant telescope, even though it was not designed for scientific use.

Théophile Moreux Théophile Moreux (20 November 1867 – 13 July 1954) was a French astronomer and meteorologist. Life Moreux was born at Argent-sur-Sauldre, Cher on 20 November 1867. He initiated the Bourges Observatory at the seminary St Célestin at Bourges, ...

(1867–1954) observed

sunspot Sunspots are phenomena on the Sun's photosphere that appear as temporary spots that are darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibit convection. Sun ...

s through the telescope and made drawings of them. And

Eugène Michel Antoniadi Eugène Michel Antoniadi (Greek: Ευγένιος Αντωνιάδης; 1 March 1870 – 10 February 1944) was a Greek- French astronomer. Biography Antoniadi was born in Istanbul (Constantinople) but spent most of his adult life in Franc ...

(1870–1944) made several drawings of

nebula A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regio ...

e. As well several large photographs of the surface of the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

, made by Charles Le Morvan (1865–1933), were published in the ''

Strand Magazine ''The Strand Magazine'' was a monthly British magazine founded by George Newnes, composed of short fiction and general interest articles. It was published in the United Kingdom from January 1891 to March 1950, running to 711 issues, though the ...

'', November 1900.

Aftermath

The company which had organized in 1886 to build the telescope declared bankruptcy after the Expo, and the telescope was put up for auction in 1909. No buyer was found and eventually the components were scrapped. The diameter mirror is on display at the Observatoire de Paris, and two of the lenses have recently been discovered in packing crates in the basement. Poor optics were not why the telescope did not work well. The telescope was located at a fair with much

light pollution Light pollution is the presence of unwanted, inappropriate, or excessive use of artificial Visible spectrum, lighting. In a descriptive sense, the term ''light pollution'' refers to the effects of any poorly implemented lighting, during the day ...

and near a very large city with much air pollution, and near sea level. The tube was not ventilated and the air inside was likely humid. Throughout its existence, the telescope was the butt of many derisive jokes and unflattering cartoons. In part this was due to the belief of the academic community that the telescope would be completely useless. As the centrepiece of an exhibition showcasing the best of the recent advances in industry and technology, it nonetheless served a purpose.Launay, pp. 461–462.

Bibliography

* Paul Gautier, “Note sur le sidérostat à lunette de 60 m de foyer et de 1,25 m d’ouverture,” in ''Annuaire du Bureau des Longitudes pour 1899'' (Paris, 1898), pp. C1–C26. * Françoise Launay, “The Great Paris Exhibition Telescope of 1900”, ''Journal for the History of Astronomy'', 38 (2007), 459–475.

References

{{Authority control Optical telescopes

Telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...

1900 in France 1900s in Paris 1900 in science Great refractors