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Astrophotography is a specialized type of photography that entails making photographs of astronomical objects in the night sky such as planets, stars, and deep sky objects such as star clusters and galaxies.
Overview Astrophotography is used to reveal objects that are too faint to observe with the naked eye, as both film and digital cameras can accumulate and sum photons over long periods of time. Astrophotography poses challenges that are distinct from normal photography, because most subjects are usually quite faint, and are often small in angular size. Effective astrophotography requires the use of many of the following techniques: Guiding With few exceptions (notably the sun, the moon, the brightest stars, the brightest planets and rare transient phenomena such as bright comets and meteors), celestial objects are typically very faint. In order to obtain good photographic images, one must therefore use long exposure times and highly sensitive film or CCDs. Because of the Earth's rotation, all celestial objects show an apparent motion over the sky, and long exposure times inevitably require some sort of compensation to avoid blurred images. Telescopes equipped with motorized equatorial mounts can compensate the Earth's rotation but need a precise control of motor speed. To achieve the best possible compensation, a separate telescope ("guidescope") attached to the same mount as the camera, or an off-axis guider integrated into the telescope used for photography is used. Both these devices make use of a reference star that is observed in a crosshair eyepiece and whose deviation from the ideal position is continually used to correct the motor speed. Advanced systems employ "autoguiding", which automates this process by using a separate CCD with which a tracking star is monitored and which controls, through a specialized electronic system, the mount's motors. History The first astrophoto is attributed to John William Draper, who took a photo of the moon in 1840. His son, Henry Draper, later became the first person to photograph the Orion Nebula in 1880, which was essentially the first deep sky astrophoto. CCD Imaging Since the early 1990s, most professional observatories switched to using CCD devices for astronomical imaging. They have numerous advantages, with increased sensitivity and more linear response. The principle disadvantage is a significant reduction in the field of view. CCDs require specific modifications for best effect in the low light conditions of astronomy, such as: Emulsion based astrophotography remains popular with amateur astronomers, but even here CCD imaging is becoming relatively cheap and is starting to replace film photography. Amateur astrophotography Although the description above suggests that astrophotographs can be made only with expensive equipment by observatories or photographers with extensive experience, in fact surprisingly good quality, wide-angle photographs of the night sky can be made by almost anyone using readily available single lens reflex 35 mm traditional film or digital cameras. Such photographs are usually made far away from the light-polluted skies of major cities or towns. This ensures that the sky is dark enough so that the photograph will not be completely washed out and ruined by bright urban light pollution. Simple wide-angle astrophotographs of constellations containing familiar star patterns (such as Ursa Major, Orion, Sagittarius and others) can be made in such a manner. Photographs using exposures of minutes or hours will show long star trails (because of the Earth's rotation). Some astrophographers do this on purpose for the desired effect. Others will try to avoid it by either using a short exposures on a stationary mounted camera, or by using a motor-driven telescope mount, in order to keep the stars as points of light in the final photograph. Digital astrophotography With the advent of consumer digital cameras featuring CCD chips more sensitive than film, astrophotography no longer requires such long exposure times, special tracking equipment or non-light polluted skies. Nothing more is required than a tripod, self-timer function and camera with manual exposure control. The photograph of Cassiopeia to the right was taken at 1/16th of a second, aperture of 3.4 and ISO equivalent film speed of 200, with no additional processing. With digital images however, it is a simple matter to brighten pictures and increase the contrast. More sophisticated techniques involve capturing multiple images to composite together in an additive process (negating tracking issues and bringing out dim objects), as well as using image processing to filter out light pollution and subtracting a "dark frame" to remove thermal noise. (The last some digital cameras do automatically for long exposures.) Ironically, unlike typical digital photography where instant results are displayed, digital astrophotography often incorporates enough post-processing that the final results won't be known until later (such as the case of an object only as bright as the background noise). Therefore it becomes useful to bracket exposures as per traditional film photography. See also Categories | ||||||||
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