Great Paris Exhibition Telescope of 1900
The Great Telescope, installed, from the ocular lens end
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| Coordinates | 48°52′N 2°17′E / 48.86°N 2.29°ECoordinates: 48°52′N 2°17′E / 48.86°N 2.29°E |
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| Telescope style |
optical telescope 
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| Focal length | 57 m (187 ft) |
 Location of Great Paris Exhibition Telescope of 1900
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The Great Paris Exhibition Telescope of 1900, with an objective lens of 1.25 m (49 in) in diameter, was the largest refracting telescope 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. 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 telescope had two interchangeable objective lenses (for visual and photographic use respectively) 1.25 m (49 in) in diameter, with a focal length of 57 m (187 ft). 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 movable plane mirror 2 m (6.6 ft) in diameter, mounted in a large cast-iron frame at the objective lens end of the telescope. The horizontal steel tube was 60 m (200 ft) 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.
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.
The telescope was erected in the Palais de l’Optique on the Champ de Mars, near the Eiffel Tower. The tube, oriented north-south, was made up of 24 cylinders 1.5 m (59 in) in diameter and rested on 7 concrete and steel pillars; its axis was 7 m (23 ft) above the floor. The room at the end which housed siderostat with the mirror had a movable dome to allow direct access to the sky.
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