The Maria Mitchell Observatory Plate Collection as a
Mirror of the Evolution of Astronomical Photographic Emulsions
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A. Davis
Maria
Mitchell Observatory, 3 Vestal Street, Nantucket, MA 02554
Received
June 21, 2004; revised September 13, 2004
Abstract The recently digitized astronomical plate
collection of the Maria Mitchell Observatory (MMO) was used to study the
evolution of limiting stellar magnitudes of the plates as various astronomical
emulsions were used over the years. The information on limiting magnitudes was
obtained by eye photometry of the weakest detectable stars on the digitized
images of the open cluster NGC 6819. In total, 174 plate copies containing NGC
6819 and covering the period of time from the 1920s to about 1990 were
evaluated. A remarkable increase of the limiting magnitude during these seven
decades was revealed. The best plates with the early astronomical emulsions,
Speedway, Presto, and HiSpeed, showed limiting magnitudes 14.4, 15.7, and 16.4,
respectively, whereas the limiting magnitude on the best plates with the
post-1950s emulsions, 103aO and IIaO (some hyper-sensitized with N2), are
17.4 and 17.2, respectively. The scatter of the limiting magnitudes from plate
to plate gradually decreased from about three magnitudes in the 1920s to about
one magnitude in the 1980s. The scatter of the limiting magnitudes must be due
not only to the scatter of the conditions of observations, but also to the
quality and homogeneity of the batches of plates, which appears to steadily
increase with time, together with the sensitivity of the plates.
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