2 0 0 9 P a r i s / M e u d o n

IWCMO Conference

Audouin DOLLFUS: The First Pic du Midi Photographs of Mars in 1909

Professor Audouin DOLLFUS prepared another talk to be given at the IWCMO, but unfortunately he could not attend because of illness the days. Here so given is an unspoken draft with figures by courtesy of Professor Dollfus


n early September 1909, atop the Pic du Midi mountain, the Baillaud brothers had just made operational the new dome and its equatorial mounting. The Count Aymar de la Baume Pluvinel (1860-1929) and Fernand Baldet (1885-1962) reached the summit by climbing the mountain on foot with their 祖aravan supplying the site. Their purpose was to study the surface of Mars by photography: the first use of the Pic du Midi telescope.


De la Baume Pluvinel, aristocrat, bachelor and wealthy, clever scientist had his own research laboratory in his castle at Marcoussis. He was an expert in the study of the solar corona at eclipse. For that purpose he was in Guyana in 1888, in Greece in 1890, Senegal in 1893, Spain in 1900, at Sumatra in 1901, in Cairo the same year, in Spain again in 1905 and he would be in the Crimea in 1915. He had also worked at Janssen痴 observatory atop Mont Blanc and had just accomplished a notable spectroscopic study of comet Morehouse at Flammarion痴 Juvisy Observatory together with Fernand Baldet.


The year 1909 had seen the planet Mars at its minimum distance from Earth. The opportunity favoured a study of the enigmatic markings and features observed upon its surface, and their variations with time. Moreover, it was also the moment to clarify the nature of the faint linear canals advocated by the Italian Giovanni Schiaparelli in 1877 and documented by the American Percival Lowell. The mystery was an exciting one, and triggered the enthusiasm of Camille Flammarion. One method of study involved making photographs of the planet.


Some attempts had been already made during the past perihelic opposition of 1907. Low in the European skies, but high in the southern countries, the planet was photographed from the Arequipa station of the Harvard observatory in Peru. Furthermore, a remarkable expedition was directed by Percival Lowell in the Taracapa-Acatama desert region of Chile, at an altitude of 1400 m, where rain never falls. The powerful 45 cm diameter Amherst observatory refractor was loaned by David P. Todd and moved to the site, where it was safely left in the open air. In June 1907, Percival Lowell, David Todd and Earl C. Slipher arrived at the site with seven tons of material. 14,000 photographic images of Mars were recorded. Slipher selected the twenty best, showing the planet at all longitudes. Some of these images apparently showed canals, the purpose of the expedition. But Lowell, to advertise the result, committed the fatal error of publishing photographs upon which the canals had been redrawn enhanced in order to be made more obvious, which disqualified the demonstration.


The expedition to Pic du Midi, no less daring, preserved all its relevance. At the top of the mountain, de la Baume Pluvinel and Baldet settled into the new ten-metre diameter dome holding the equatorial mounting carrying jointly a 50 cm Newtonian reflector and a 23 cm refractor (the latter 6 m long). The telescope power was at least competitive with LowellChile 1907 instrument, but located at a high altitude site on the top of a mountain. The use of the instrument was apparently not easy. The observer had to be on a small platform at the top of the tube and aside the opening of the dome, in the dark and in the wind. The guiding of the telescope was operated at the eyepiece of the refractor, at the lower end of the tube. But perfect good weather arrived and stayed for a long time. Such is a property of the Pic du Midi: after a long period of bad weather, the sky at once clears up, and then it remains as such for several weeks. The telescopic seeing reaches perfect steadiness. Each night (in the second part of the night, essentially), the images are rock steady.


Contrary to Slipher, whom in Chile recorded long sequences of continuous photographic shots all night long, de la Baume Pluvinel and Baldet chose to watch the seeing at the eyepiece of the telescope, and to shoot only at the best moments, about twenty times on the same plate. The operation needed about fifteen minutes. Then, the plate was processed in the darkroom and submitted to a quick scrutiny. The same operation was repeated again one hour later. Meanwhile, the planet had rotated by about 15ー in longitude. After six hours of such observations, Mars has displayed 90ー of its surface. In addition, some images were recorded through a blue filter. Next night, the planet having a period of rotation of 24h 37m - slightly slower than the Earth痴 - the planet is seen with a shift of about 10ー in longitude. After a few weeks, the same presentations in longitude recur, making it easy to make comparisons and to analyse the variations in the Martian surface configurations.


The observations ended on 20 October 1909. A total of 79 plates were collected, among which the two astronomers selected 49 of the best. They processed a high contrast positive copy of the best image from each of these plates. A large scale display of these 49 selected images enabled easy analysis. Later, the astronomers of Meudon observatory reprocessed the plates with the 祖omposite technique advocated by Bernard Lyot in 1941: For each plate, the eight or ten best images are selected and stacked under the enlarger, to produce a single averaged image. The graininess of the image is reduced, small defects are washed out, and the sharp faint features are enhanced. The IAU Planetary Photography Centre at Meudon processed the entire sets of planetary images recorded by observatories throughout the world brought together for the purpose. In 1909, during the period of the Pic du Midi observations, good images of Mars were also obtained by E. C. Slipher at Lowell observatory, Arizona. They do not always attain the sharpness of the Pic du Midi plates. At Yerkes observatory, the skilful observer E. E. Barnard produced a few exquisite images which, when processed with the 祖omposite technique, show subtle details.


As a first result of the Pic du Midi operation, the photographic images did not show 祖anals upon the surface of Mars. At the same time, Eugene Antoniadi was observing visually with the large 83 cm diameter Meudon observatory refractor. In perfect seeing conditions, he also noted the absence of any features which could not be regarded as natural.


For another result, the two astronomers were surprised to discover that, on the images taken with the blue filter, the features of the surface of Mars are no longer visible. Only the white patch of the polar cap can be seen. The same effect was recognized upon photographic images taken at Meudon observatory with the large refractor. This peculiar effect was also discovered independently by E. C. Slipher at Lowell observatory. Later, when observing again during the next periods of close approach of Mars with the Earth, the American astronomer noted some circumstances during which the Martian surface configurations seemed to reappear in blue light. He advocated for this the presence of a 礎lue haze in the planetary atmosphere, which was veiling the surface but occasionally able to dissolve, the 礎lue clearing, making the markings reappear. Conversely, the astronomers at Meudon observatory demonstrated that the dark features disappear in blue because the lighter areas of the surface (which are yellow in colour) darken sharply toward blue wavelengths and equal the albedo of the dark features in blue light. Occasionally, some white hazes or mists occur, preferentially over the bright continents, reproducing the surface configurations when observed in blue light.


The 1909 Pic du Midi plates have also added their contribution to the global study of the evolution of the Martian surface configurations conducted at Meudon observatory on the basis of all photographic images of Mars taken throughout the world from 1907 to 1971. It was concluded that the shape and contrast variations are not due to vegetation, as was frequently suggested at the time, but are related to the transportation and deposition of dust under the effects of winds, at the places and during the periods when the heating of the ground by solar insolation is at maximum.


Such were the results of these exciting 1909 observations, which marked the start of astronomical research at Pic du Midi.



This is an English translation by A DOLLFUS himself of the paper: 1909; PREMIERES PHOTOGRAPHIES DE MARS AU PIC DU MIDI

Publi dans l但stronomie, Novembre 2009, pp. 27-30.


Audouin DOLLFUS, Meudon Observatory


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