2004JAVSO..32..126D

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-1950 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.