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Andrew Ainslie Common (1841—1903) astronomer and astronomical photographer


reciprocity failure


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The application of photography in astronomy. Images can build up for long periods on a photographic emulsion or charge-coupled device (CCD), revealing stars and other objects invisible to the eye, and are recorded permanently, allowing their positions and brightnesses to be measured accurately.

Astrophotography began in earnest in 1883, when A. A. Common photographed the Orion Nebula with a 36-inch (0.91-m) reflector, recording stars which could not be seen visually with the same instrument. Astronomers were quick to exploit the new medium, and for the next hundred years it was the primary means of making optical observations. Its application to spectroscopy was equally important.

In astrophotography, a photographic emulsion or CCD is placed at the focus of a telescope, instead of an eyepiece. The emulsion may be on a glass plate or film. A shutter is required for making the exposure, which can last many minutes or even hours. During the exposure the telescope must be guided accurately on the object being photographed (see Guider).

Conventional emulsions suffer from reciprocity failure, which limits effective exposure times to less than 30 min. Emulsions specifically designed for astrophotography are available with reduced reciprocity failure. Alternatively, conventional emulsions can be hypersensitized to improve their long-exposure performance.

Colour photography is possible using conventional colour film, but the best results with long exposures come from three-colour photography. Separate exposures are made on hypersensitized black-and-white emulsions, each filtered to record only red, green, or blue light. The separate exposures are then combined in the darkroom to give a full-colour picture.

Photographic emulsions have now been almost entirely replaced for professional purposes by CCDs, which have a greater and more linear sensitivity to light.

Subjects: Astronomy and Astrophysics.

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