ASTRONOMICAL PHOTOGRAPHY [Film Technology]

astronomical photography 
[Film Technology]

     In no other field of science has photography played a more important role than in astronomy. By placing the photographic plate in the focal plane of a telescope, astronomers can obtain precise records of the locations and brightness of celestial bodies. By comparing photographs of the same region of the sky taken at different times, proper motions of certain objects such as comets can be detected. An important quality of the photographic plate for astronomy is its ability to record, by means of long time-exposures, astronomical objects too faint to be observed with the naked eye.  Recently, the sensitivity of photographic recording has been improved by image-enhancing techniques. In a process known as the photoelectric effect starlight liberates electrons on a photocathode that is placed in the focal plane of the telescope. The liberated electrons are directed to a photographic plate to form the image. Computer enhancement techniques create sharper, more detailed images from sometimes fuzzy and distant photographs from outer space. Computers digitize the photographic information and then reproduce it with greatly improved resolution. In a further refinement, charge-coupled devices (CCDs) dispense with a photographic plate altogether; individual photons are recorded electronically, and are distributed by a microprocessor along a series of picture elements (pixels) which when built up in rows, form an image which can be manipulated digitally by computer. The pixel in these pictures is thus analogous to the silver-halide grain in conventional photography in governing the resolution of the image. CCDs are the imaging technology used by the Hubble Space Telescope and space exploration probes such as Galileo, and are the prime system used by the world's major terrestrial telescopes, such as the Keck Telescope. The reason for their dominance in this area is their superiority in registering very faint astronomical objects; their ability to record individual photons gives tham a light-collecting power far greater than even the most sensitive film.

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