Photographic plate
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.
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 Wolf beginning with his discovery of
323 Brucia in
1891. The first
natural satellite discovered 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 Hess discovered, in the
1910s,
cosmic radiation as it left traces on stacks of photographic plates, which he for that purpose brought up on high mountains or let mount 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 imaging and
material science applications, although they have been largely replaced with reusable and computer readable
image plate detectors 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
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 3817115997
*
film base*
X-ray*
Camera*
The Sonneberg Plates Archiv (Sonneberg Observatory)
