15 Reseaux plates for measuring astronomical photographs.
In 1887 astronomers from around the world embarked a massive new enterprise; known as the Carte du Ciel (Mapping the Stars) project it involved photographing and measuring the stars in both hemispheres. Australia was actively involved in project with observatories in Sydney, Melbourne, and Perth keen to participate in this international project. Each observatory was allocated a zone of the sky and was expected to record it using instruments of a standard pattern.
Originally all the photographic plates were to be measured at a central plate-measuring facility in Paris but when this failed to materialise Melbourne and Sydney set up a shared facility located in Melbourne. Some of these reseaux made by P. Gautier were sent to Melbourne sometime after July 1891 and were used in conjunction with the photographic plates taken during the 'Mapping the Stars' project.
For the 'Mapping the Stars' project all the photographic plates were coated with a dry gelatine emulsion introduced to Australia around 1883. It was feared that the emulsion sometimes shrank after being developed making precise measurements of the plate difficult. To overcome this problem, and to make measuring easier, a glass plate the same size as the photographic one was coated in silver and ruled into fine lines five millimetres apart.
This plate (or reseau) was then placed in the dark slide in direct contact with photographic plate from the astrographic camera and exposed for a second or two to imprint the lines on the emulsion. The resulting image, traversed by a series of very fine lines, could then be used to measure the plates with respect to the lines. As it turned out shrinkage was minimal but the lines were indispensable as reference marks in determining the placement of the stars.
By 1915 there was a huge backlog of unmeasured photographs and The Sydney Observatory purchased their own plate measuring machines to try to clear the backlog. It is likely that this reseau and dark slide came from Melbourne to Sydney Observatory around this date. Once in Sydney they were used to continue the work on measuring the photographic plates taken by the observatory.
Todd, David, P., Stars and Telescopes, Sampson Low, Marston, and Co., 1900
Glass, I. S., Victorian Telescope Makers, the Lives and Letters of Thomas and Howard Grubb, Institute of Physics Publishing, Bristol and Philadelphia, 1997
Ellery, R.L.J., 'Photographic Charting of the Heavens', cited in Proceedings of the Royal Society of Victoria, Volume IV, part 1, Stillwell and Co., Collins Street, Melbourne, 1892
Turner, H. H., The Great Star Map, John Murray, London, 1912
Significance Statement by Geoff Barker, August, 2007