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Concept
Photographic magnitude
Summary
Photographic magnitude (mph or mp ) is a measure of the relative brightness of a star or other astronomical object as imaged on a photographic film emulsion with a camera attached to a telescope. An object's apparent photographic magnitude depends on its intrinsic luminosity, its distance and any extinction of light by interstellar matter existing along the line of sight to the observer.
Photographic observations have now been superseded by electronic photometry such as CCD charge-couple device cameras that convert the incoming light into an electric current by the photoelectric effect. Determination of magnitude is made using a photometer.
Prior to photographic methods to determine magnitude, the brightness of celestial objects was determined by visual photometric methods. This was simply achieved with the human eye by compared the brightness of an astronomical object with other nearby objects of known or fixed magnitude : especially regarding stars, planets and other planetary objects in the Solar System, variable stars and deep-sky objects.
By the late 19th Century, an improved measure of the apparent magnitude of astronomical objects was obtained by photography, often attached as a dedicated plate camera at the prime focus of the telescope. Images were made on orthochromatic photoemulsive film or plates. These photographs were created by exposing the film over a short or long period of time, whose total exposure length accumulates photons and reveals fainter stars or astronomical objects invisible to the human eye. Although stars viewed in the sky are approximate point sources, the process in collecting their light cause each star to appear as small round disk, whose brightness is approximately proportional to the disk's diameter or its area. Simple measurement of the disk size can be optically judged by either a microscope or by an specially designed astronomical microdensitometer.
Early black and white photographic plates used silver halide emulsions that were more sensitive to the blue end of the visual spectrum.
Official source
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