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
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I |
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
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
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
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
Contrary to Slipher, whom in 
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
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 
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
Publi dans l但stronomie, Novembre 2009, pp. 27-30.
Audouin DOLLFUS, Meudon Observatory