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A. Davis (SUNY at Plattsburgh & Maria Mitchell Obs.), V. Strelnitski (Maria Mitchell Obs.)
In November of 2002, the 1.5-year long project of digitizing the entire 8,000+ photographic plates of the Maria Mitchell Observatory's (MMO's) plate collection was completed. The plates were taken over the years from 1913 to 1995 with the MMO's 7.5-inch Cooke/Clark camera. The goal of the present project was to study the limiting stellar magnitudes of the plates as various astronomical emulsions were used over the years. Most information on limiting magnitudes was obtained by eye photometry of the weakest detectable stars on the digitized images of the open cluster NGC 6819. One hundred and seventy-four plates containing NGC 6819 and covering the period of time from the 1920's to about 1990 were evaluated. We reveal a remarkable gradual increase of the limiting magnitude, obtained with the same exposure time, during these 7 decades: 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's emulsions, 103aO and IIaO (some hypersensitized with N2), are 17.4 and 17.2, respectively. The dispersion of the limiting magnitudes from plate to plate gradually decreased from about 3.5 magnitudes in the 1920's to 1.5 magnitudes in the 1980's. We conclude that the limiting magnitude dispersion is due not only to the dispersion of the conditions of observations, but also to the quality and homogeneity of the batches of plates, which appears to dramatically improve with time. This project was supported by the NSF/REU grant AST-0097694 and the Nantucket Maria Mitchell Association.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.