- Photographic plate
Photographic plates preceded
photographic filmas a mean of photography. A light-sensitive emulsionof silversalts was applied to a glassplate. This form of photographic material 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 lightreceived.
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 ObservatorySky Survey (POSS) of the 1950s, the follow-up POSS-II survey of the 1990s, and the UK Schmidt survey of southern declinations. A number of observatories, including Harvard Universityand Sonneberg Observatory, maintain large archives of photographic plates, which are used primarily for historical research on variable stars.
Many solar system objects were discovered by using photographic plates, superseding earlier visual methods. Discovery of
minor planets using photographic plates was pioneered by Max Wolfbeginning with his discovery of 323 Bruciain 1891. The first natural satellitediscovered using photographic plates was Phoebe in 1898. Pluto was discovered using photographic plates in a blink comparator; its moon Charon was discovered by carefully examining a bulge in Pluto's image on a plate.
Photographic plates were also an important tool in early
high-energy physics, as they get blackened by ionizing radiation. For example, Victor Franz Hessdiscovered, in the 1910s, cosmic radiationas it left traces on stacks of photographic plates, which he left for that purpose on high mountains or let into the even higher atmosphere using balloons.
The sensitivity of certain types of photographic plates to ionizing radiation (usually
X-rays) is also a useful in medical imagingand material scienceapplications, although they have been largely replaced with reusable and computer readable image platedetectors and other types of X-ray detectors.
Use of photographic plates has declined significantly since the early 1980s, 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 detecting area and resolution of most photographic plates, which has forced modern survey cameras to use large arrays of CCD chips. The longevity of electronic data and data formats (such as FITS) is also uncertain.
* 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 3-8171-1599-7
* [http://stw-serv.stw.tu-ilmenau.de/science/plate/index_E.html The Sonneberg Plates Archiv (Sonneberg Observatory)]
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