5.6 Carte du Ciel
85
(1844–1931). The emphasis on uniformity not only proved difficult to implement in practice, but also made the whole project unnecessarily rigid. Twenty
observatories participated, none from the USA. According to Hendricus van
de Sande Bakhuyzen, the Sterrewacht Leiden could not afford the purchase of
the telescope and the materials for the execution, and did not contribute.
The production of the Astrographic Catalogue was not without problems.
Some observatories were relatively fast, others took much longer to take the
plates. The intention was that all exposures were taken three times on the
same plate, each time with a slight shift of the direction in which the telescope pointed to the sky. In this way, stars could be distinguished from plate
faults; and because the three observations had different exposure times, effects
of overexposure were also under control. But all of this was a lot of work and
measuring up the plates was extremely time consuming. The quality requirements were very high, and rightly so. The project comprised 22,000 plates;
the catalog was only completed in 1961. However, completing is a misleading word, because most published volumes only contained measurements in
rectangular coordinates on the plates, and it still was a huge job to convert
these into right ascensions and declinations. Still, this exercise turned out not
a wasted effort. When the European Space Agency launched the astrometric
satellite Hipparcos in 1989, which carried out accurate measurements of parallax and proper motions of over a hundred thousand stars until 1993, 4 the
Astrographic Catalogue was digitized and reduced to proper coordinates and
made available electronically and was used as a ‘first-epoche’ measurement for
the proper motions. As a result, in many cases the time interval over which
proper motions could be measured increased to almost a century.
The second part of the catalog was a ‘real’ Carte du Ciel, i.e. a map of the
sky in the form of photographic images. In the end, the idea was to produce
these as copper engravings, but that soon turned out to be far too expensive.
This part never got very far. In addition, a similar project was carried out
around 1950, at least for the northern hemisphere, in the form of the National
Geographic Society—Palomar Observatory Sky Survey. This was produced
with a special telescope, the 48 in. (1.2-m) Schmidt telescope at Palomar
Observatory in California. It was specially designed to take photographs of
large pieces of sky. The Sky Survey was produced in pieces of sky measuring
6 ◦ × 6 ◦ (with the Carte du Ciel it would have been 2 ◦ × 2 ◦ , so almost ten
times smaller fields) on photographic plates of 14 × 14 inches (35.5 cm). The
exposures date for the greater part from between 1956 and 1958 and were then
printed on paper. Every astronomical institute still has a copy of these; the Sky
Survey was very affordable. The southern sky was mapped in the 1970s with
4 Later a catalog was published based on Hipparcos observations of 2.5 million stars, with greater accuracy.
85
(1844–1931). The emphasis on uniformity not only proved difficult to implement in practice, but also made the whole project unnecessarily rigid. Twenty
observatories participated, none from the USA. According to Hendricus van
de Sande Bakhuyzen, the Sterrewacht Leiden could not afford the purchase of
the telescope and the materials for the execution, and did not contribute.
The production of the Astrographic Catalogue was not without problems.
Some observatories were relatively fast, others took much longer to take the
plates. The intention was that all exposures were taken three times on the
same plate, each time with a slight shift of the direction in which the telescope pointed to the sky. In this way, stars could be distinguished from plate
faults; and because the three observations had different exposure times, effects
of overexposure were also under control. But all of this was a lot of work and
measuring up the plates was extremely time consuming. The quality requirements were very high, and rightly so. The project comprised 22,000 plates;
the catalog was only completed in 1961. However, completing is a misleading word, because most published volumes only contained measurements in
rectangular coordinates on the plates, and it still was a huge job to convert
these into right ascensions and declinations. Still, this exercise turned out not
a wasted effort. When the European Space Agency launched the astrometric
satellite Hipparcos in 1989, which carried out accurate measurements of parallax and proper motions of over a hundred thousand stars until 1993, 4 the
Astrographic Catalogue was digitized and reduced to proper coordinates and
made available electronically and was used as a ‘first-epoche’ measurement for
the proper motions. As a result, in many cases the time interval over which
proper motions could be measured increased to almost a century.
The second part of the catalog was a ‘real’ Carte du Ciel, i.e. a map of the
sky in the form of photographic images. In the end, the idea was to produce
these as copper engravings, but that soon turned out to be far too expensive.
This part never got very far. In addition, a similar project was carried out
around 1950, at least for the northern hemisphere, in the form of the National
Geographic Society—Palomar Observatory Sky Survey. This was produced
with a special telescope, the 48 in. (1.2-m) Schmidt telescope at Palomar
Observatory in California. It was specially designed to take photographs of
large pieces of sky. The Sky Survey was produced in pieces of sky measuring
6 ◦ × 6 ◦ (with the Carte du Ciel it would have been 2 ◦ × 2 ◦ , so almost ten
times smaller fields) on photographic plates of 14 × 14 inches (35.5 cm). The
exposures date for the greater part from between 1956 and 1958 and were then
printed on paper. Every astronomical institute still has a copy of these; the Sky
Survey was very affordable. The southern sky was mapped in the 1970s with
4 Later a catalog was published based on Hipparcos observations of 2.5 million stars, with greater accuracy.