Carte du Ciel
Twenty observatories from around the world participated in exposing and measuring more than 22,000 (glass) photographic plates in an enormous observing programme extending over several decades.
Despite, or because of, its vast scale, the project was only ever partially successful – the Carte du Ciel component was never completed, and for almost half a century the Astrographic Catalogue part was largely ignored.
As a result of the Astrographic Congress of more than 50 astronomers held in Paris in April 1887, 20 observatories from around the world agreed to participate in the project, and two goals were established: For the first, the Astrographic Catalogue, the entire sky was to be photographed to 11 mag to provide a reference catalogue of star positions that would fill the magnitude gap between those previously observed by transit and meridian circle instrument observations down to 8 mag – this would provide the positions of a reasonably dense network of star positions which could in turn be used as a reference system for the fainter survey component (the Carte du Ciel).
A contemporary account of this vast international astronomical collaboration, published in 1912, was given by Herbert Hall Turner, then Savilian Professor of Astronomy at Oxford University.
Around half of the observatories ordered telescopes from the Henry brothers (Paul and Prosper) in France, with others coming from the factory of Howard Grubb of Dublin.
[4] These telescopes were termed normal astrographs with an aperture of around 13 inches (33 cm) and a focal length of 11 feet (3.4 m) designed to create images with a uniform scale on the photographic plate of approximately 60 arcsecs/mm while covering a 2° × 2° field of view.
A more serious problem was that while many European astronomers were preoccupied with this project, which required steady, methodical labor rather than creativity, in other parts of the world notably the United States astrophysics was becoming far more important than astrometry.
The charts proved to be excessively expensive to photograph and reproduce, generally via engraved copper plates (photogravure), and many zones were either not completed or properly published.
The vast amount of work invested in the Astrographic Catalogue, taking plates, measuring, and publishing, was looked at for a long time as giving only a marginal scientific profit.
The data was then reduced anew (at the US Naval Observatory in Washington under the leadership of Sean Urban), using the reference stars measured by the Hipparcos astrometry satellite.
[6][7] Sean Urban of the US Naval Observatory wrote in 1998:[5] The history of the Astrographic Catalogue endeavour is one of dedicated individuals devoting tedious decades of their careers to a single goal.
