The great paris exhibition telescope of 1900

La grande lunette de l'exposition de 1900

It's largest telescope for moon and sun observation at that time

 It consists of:

a siderostat

 

a long steel tube/tunnel of 60 m long

 

an objective holder ( Francois Deloncle)

drawing of objectives:

the objective:

from wikiwand:

The telescope had two interchangeable achromatic doublet lenses 1.25 m (49.2 in) in diameter, one intended for visual observation and the other for taking photographic shots - the device functioning as an astrograph -, and had a focal length of 57 m (187 ft). 

Due to its very large size, the scope was mounted in a fixed horizontal position. Given the size of the instrument, the use of an equatorial mount proving impossible, the designers designed a Foucault siderostat mounted on a cast iron frame, all specially adapted for the telescope.

 Light from astronomical objects was redirected into the optical tube through the siderostat which had a moving mirror 2 m (6.56 ft) in diameter. As for the horizontal steel tube, it was 60 m (197 ft) long. The telescope eyepiece (or photographic plate), set on rails, could be shifted five feet for focusing. With the lowest power of 500×, the field of view was three minutes of arc.

The biggest attraction should be the large siderostat, with superior flatness. Only with such flatness, this is possible to reflect light via such a long tube / tunnel.

The large 1.25 m objectives, however photos of it cannot be found on the internet, and cast doubts of its claimed dimensions and quality. Some said it was waiting polishing and never completed.

In addition, if these objectives are polished well as it should, it would serve much better as front objective of a normal all purpose telescope to observe not just sun and moon

Some observation drawings from the telescope:

Scientific American:

With no serious buyers, the telescope was eventually dismantled, and most of the parts were sold as scrap metal. The large mirror, however, was preserved and is presently on display at the Observatoire de Paris, the largest astronomical research center in the world which has been open since 1667 and houses the discoveries of the finest astronomers in France, while two of the lenses are packed away in the basement of the museum. 

Mirror now in Paris Observatory(TBC):

OTOH, during that era, however, Kew solar telescopes were more commonly used to take solar images:

In this design, you do not need to have big front objective, image is formed by projection to a translucent screen.

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