1887 Director of Sydney
Observatory Henry C. Russell
attends the first International
Astrophotographic Congress
in Paris on the 16th April. An
ambitious project is launched
to map the entire celestial
sphere. Sydney and Melbourne
observatories invited to take part.1890
THE MAN FROM SNOWY
RIVER PUBLISHED
HISTORY IN
THE MAKING
Through the decades
of the Astrographic
Catalogue project, the
world went from horse
and cart to the moon.THE GREAT STAR CATALOGUE
At Paris Observatory in 1887 a remarkable meeting
of astronomers from around the world produced
a startling idea – to create a map of the skies using
the latest photography and measuring techniques.
It would capture the position of the stars in both
northern and southern hemispheres.
The task of compiling the work would fall to
18 observatories around the world. The sky was divided
into zones, allocated to different observatories. In
Australia, observatories in Melbourne and Sydney
were the first to take part. Adelaide came on board
shortly after to assist Melbourne; Perth joined the
effort in 1900, taking over the zones originally assigned
to Rio de Janiero Observatory. Together the Australian
contingent would go on to map 43% of the southern
hemisphere – one of the richest concentrations of stars
because the Milky Way galaxy crossed right through it- and 17% of all the stars recorded.
The AC aimed to record the position of all the
stars, and the Carte sought to be one of the most
detailed photographic charts of the heavens. Each
would require separate glass plates. The plan was to
make photographic charts of all stars down to the 14th
magnitude (some 40 million stars) and catalogues of
the positions of stars down to the 11th magnitude (two
to three million), based on accurate measurement of
the photographs.
Coated with a special dry gelatine emulsion, the
stellar photography plates were first exposed to a
réseau plate – the same size as the photographic plate,
but coated in silver and ruled into a grid of fine lines
five centimetres apart. The réseau plate was placed in
direct contact with the photographic plate and exposed
for four to five minutes to imprint the réseau grid on
the emulsion. The result was a photographic slide
traversed by a grid of very fine lines – indispensable as
reference marks in determining the placement of the
stars. The plates were then ready for star photographs.
Male astronomers took the photographs using
special “astrographic telescopes”. Every four-to-six-
minute exposure was arranged so that a plate would
show a number of standard stars, from which the rest of
the stars on the plate – varying from 300 to 1000 – could
be measured. In Australia, most of the photographs
were taken by 1915; some were later retaken. Each
contained hundreds, if not thousands, of stars.
Ensuring precise measurements was an intricate
process. Originally the plates were to be measured
at a central facility in Paris, but a change of plans saw
the work transferred to individual observatories;
Melbourne and Sydney shared a measuring bureau
located in Melbourne.
The women who observed, catalogued and
calculated the star positions on the AC had
outstanding mathematical ability, with the analytical
skill to measure and calculate the stars’ positions. They
worked on purpose-built measuring machines called
micrometers and used standard astrometry reduction
formulas to determine the true star co-ordinates.
They worked in pairs, exchanging roles to measure
each star on each photographic plate twice – once on
the front face and again when the slide was turned
180 degrees. The photographic plate was placed
in a micrometer and the star’s X and Y co-ordinates
were measured and calculated. The measurer on
the micrometer would read out the data, which
was entered into logbooks by the second measurer.
Afterwards the mean of the two measurements was
calculated and also entered into the logbook.
Co-ordinate measurements were compared
between measurers in an effort to eliminate human
error. Given that the plates overlapped, each star
could be measured three or four times. Given front,
reverse and overlap measurements, each glass
plate was handled multiple times. According to
contemporary reports in the project’s early years, on
average 170 stars could be measured in an hour.1888
AUSTRALIA’S FIRST ELECTRIC
STREET LIGHT (TAMWORTH)
36 – COSMOS Issue 86ASTROGRAPHIC CATALOGUE