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Photographic plates were one of the earliest forms of photographic film, in which a light-sensitive emulsion of
silver salts was applied to a glass plate. This
form of photographic emulsion largely faded from the consumer market in the early years of the 20th century, as more convenient
and less fragile films were introduced. However, photographic plates were in wide use by the professional astronomical community as late as the 1990s. Such
plates respond to ~2% of light received.
Glass plates were far superior to film for research-quality imaging because they
were extremely stable and less likely to bend or distort, especially in large-format frames for wide-field imaging. Many famous
astronomical surveys were taken using photographic plates, including the first Palomar Observatory Sky Survey (POSS) of the 1950s, the follow-up POSS-II survey of the 1990s, and the
UK Schmidt survey of southern declinations. Many observatories, including Harvard University and Sonneberg Observatory,
maintain large archives of photographic plates, primarily for historical research on variable stars.
Use of photographic plates has declined significantly since the 1990s, replaced by charge-coupled devices (CCD). CCD cameras have several benefits over glass plates, including
highly efficient, linear response to light, and simplicity of image acquisition and processing. However, even the largest format CCDs (e.g. 8192x8192 pixels) still do not have the
resolution of most photographic plates, and the longevity of electronic data
and data formats (such as FITS) is uncertain.
Literature
- Peter Kroll, Constanze La Dous, Hans-Jürgen Bräuer: "Treasure Hunting in Astronomical Plate Archives." (Proceedings of
the international Workshop held at Sonneberg Observatory, March 4 to 6, 1999.) Verlag Herri Deutsch, Frankfurt am Main
(1999), ISBN 3817115997
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