Brass holder for glass plates
This holder was designed to hold a photographic glass plate of size 16 cm by 16 cm. It was fixed in the focal plane of the astrographic telescope with bolts, and could be easily dismounted to insert or remove the glass plates. In the holder, a fine grid of perpendicular lines was inserted with the purpose of improving the measurement of the stellar positions. Each glass plate covers a field of 2° by 2° on the sky. Because three distinct exposures were recorded on every plate, each object could be measured three times. At the same time the calibration grid was also photographed, which enabled to measure the positions of the stars detected on the plates more precisely (see also the explanation of the print on paper).
More information on the astrographic equatorial telescope which has contributed since 1908 to the mapping of the Uccle zone for the "Carte du Ciel" project and on the project see http://www.astro.oma.be/en/scientific-research/astronomy-astrophysics/carte-du-ciel/
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