Fame

The design of the Henry Brothers’ Carte du Ciel Astrograph was adopted by many of the observatories involved in the project, who commissioned them to construct similar instruments. In hindsight the Carte du Ciel was doomed to failure from the very start. Firstly, it was over ambitious and required vast amounts of resources, which the 20 participating institutions could ill afford. Secondly it had a disastrous effect on the quality and importance of the research carried out by its participants. At the beginning of the 20th century astronomical research was moving away from astrometry into the new field of astrophysics, where the major observatories in the United States such as those at Mount Wilson and Lick were making ground breaking discoveries providing clues as to the very origins of our universe. On the other hand the observatories involved in the Carte du Ciel project were bogged down in painstaking laborious detail of photographic plate measurements. Although the Carte due Ciel project was a failure and one which lasted until as late as 1962 when the last photographic plate was taken, it served as a useful lesson of how not to be over ambitious and to define achievable and realistic goals. This lesson is borne out by the great success of the much smaller Cape Photographic Durchmusterung Project of Sir David Gill which measured the relative positions and brightness of half a million southern hemisphere stars, published in three volumes between 1896 and 1900. If the truth be known the Carte du Ciel and the great idea it represented came too soon in history. The technology needed to achieve success was just not there, and in particular that needed to create ’machine readable’ photographic data. This technology was not to become realistically possible until the latter half of the 20th century. The enormous effort expended on the Carte du Ciel project was not entirely in vain, because much of its data became immensely useful in the determination of the proper motions of 2.5 million stars whose positions had been found by the Hipparcos astrometry satellite, launched in 1989.

Sponsor

Paul Pierre Henry was born on the 21st August, 1848 at Nancy, France. He and his younger brother Prosper Henry (born 10th December 1849) grew up to become two of the greatest pioneers of Astrophotography and were also responsible for the construction of some of the finest telescopes ever made. The two brothers were inseparable and worked together throughout their lives in close collaboration. Little is known of their early life save that they came from a modest family, and were educated at a local Catholic School. They began their careers as opticians working in their home town of Nancy. In 1864 they came to Paris, and began work at the Paris Observatory in the Department of Meteorology, Paul in 1864 and Prosper a year later. In 1868 they were promoted to assistant astronomers by Urbain Le Verrier, then Director of the Observatory. In 1871 Delaunay who succeeded Le Verrier as Director, recognized the Henry Brothers talent for optics and transferred them to the Department of Astronomy. In 1871 they began the task of completing the late Jean Chacornac’s charts of the Ecliptic, begun in 1852, but not completed. The work involved competing 72 charts, each 13” square covering a 50 field of view containing stars down to magnitude 13. They became alarmed at the number of stars they found in the region of the Milky Way. There were so many stars that to chart them visually was impossible. They then began experimenting with photography as a means to speed up the process. Their work on the ecliptic charts brought an added bonus; between them they found a total of 14 asteroids, the first Liberatrix discovered in September 1872. In August of 1884 their results were presented to the French Academy Sciences by Admiral Amedee Mouchez then Director of the Paris Observatory. Such was the success of their work that they commissioned to construct a 13” (33cm) Photographic Refractor. In the field of Astrophotography their greatest contribution came in 1885-6, when they were the first to take successful photographs of the planets, when they imaged Jupiter and Saturn. Prior to this time others had tried including contemporary pioneers like Warren de La Rue, but failed; his images of 1857 were only ½ mm across, and were therefore barely visible! During the course of their career the Henry Brothers continued to construct telescopes, and in collaboration with the engineer Paul Gauthier produced some of the finest ever made. In particular they were responsible for the 30” Refractor at the Nice Observatory and the great 32.7” Refractor at the Paris Observatory at Meudon. Paul Henry died on the 4th January 1905 at Montrouge on the outskirts of Paris, after suffering a thrombosis in his brain. His beloved brother Prosper had died 18 months earlier after a climbing accident whilst on holiday at Pralognan, in Savoy on the 25th July 1903. The Henry Brothers received many awards during their lifetime, but it is probably the naming of the lunar crater Henry Frères which would have pleased them the most. In recognition of the work of Paul Henry and his brother Prosper, the Minor Planet Centre, two asteroids were named after them ‘P.P. Henry’ and ‘P.M. Henry’. In life they were inseparable in death they are remembered forever on the surface of the Moon with a single crater.

Construction

In 1887, Admiral Amédée Mouchez, director of the Paris Observatory, launched a project to map the sky at an International Astronomical Congress which met in Paris where 56 participants (representing 16 countries) attended. Recognizing the potential of new photographic techniques in mapping the stars, he proposed an international campaign to map and identify the coordinates of several million stars in the celestial sphere, to an apparent magnitude of about 11 or 12. This project became known as the Carte du Ciel. In 1890 the Paris Observatory constructed its own telescope to be used for the Carte du Ciel project. The telescope constructed was an equatorial doublet by Paul Gautier, which included a photographic lens of 33 cm in diameter (focal length 344 cm) and a visual objective of 19 cm in diameter (focal length 360 cm). The optics were made by the brothers Paul and Prosper Henry. The telescope was equipped with photographic chassis which held plates of 16cm x 16cm. The completed instrument had a field of view of some 3 degrees and took photographs of stars down to magnitude 14-15 with exposures of about one hour. This work became the foundation of the ill fated Carte du Ciel project, which the Henry Brothers were great supporters. By April 1891, the list of participating observatories and distribution areas of the sky to photograph were finalized. The 18 selected observatories were equipped with instrument similar to the one built by the brothers Henry. This telescope was used the Henry Brothers to take successful images showing the nebulosity surrounding the Pleiades (M45), the Globular Cluster M13, and the Double Cluster in Perseus. The telescope can now be found in the Paris Observatory museum, housed in its original dome, together with the instrument used to measure the photographic plates.

Specification

The Henry Brothers’ Carte du Ciel Astrograph had the following Specification: Type: Equatorial Twin OTA Refractor. Aperture: Photographic 13” (33cm); Visual 7.6” (19cm) Achromatic Doublets; Optics: Paul and Prosper Henry; Focal Length: Photographic 137” (344cm); Visual 144” (360cm) Focal Ratio: f10.4 (Photographic); f18.9 visual Mount: English Equatorial by Paul Gautier; Photographic Plate: 13cm x 13cm.