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A star trail is a type of photograph that utilizes long-exposure times to capture the apparent motion of stars in the night sky due to the rotation of the Earth. A star trail photograph shows individual stars as streaks across the image, with longer exposures resulting in longer streaks. Typical exposure times for a star trail range from 15 minutes to several hours, requiring a 'bulb' setting on the camera to open the shutter for a longer period than is normal. Star trails have been used by professional astronomers to measure the quality of observing locations for major telescopes. == Capturing star trail images == Star trail photographs are captured by placing a camera on a tripod, pointing the lens toward the sky, and allowing the shutter to stay open for a long period of time. Star trails are considered relatively easy for amateur astrophotographers to create. Photographers generally make these images by using a SLR camera with its lens focus set to infinity. A cable release allows the photographer to hold the shutter open for the desired amount of time. Typical exposure times begin at 15 minutes and can be many hours long, depending on the desired length of the star trail streaks on the image. Even though star trail pictures are created under low-light conditions, the long exposure times allow for fast films, such as ISO 200 and ISO 400, to be used.〔 Wide-apertures, such as f/5.6 and f/4, are recommended for star trails.〔 Because exposure times for star trail photographs can be several hours long, camera batteries can be easily depleted. Mechanical cameras that do not require a battery to open and close the shutter have an advantage over more modern film and digital cameras which utilize battery power. On these cameras, the Bulb, or B, exposure setting is used to keep the shutter open. Another problem that digital cameras encounter is an increase in detector noise with increasing exposure time.〔 American astronaut Don Pettit recorded star trails with a digital camera from the International Space Station in earth orbit between April and June, 2012. Pettit described his technique as follows: "My star trail images are made by taking a time exposure of about 10 to 15 minutes. However, with modern digital cameras, 30 seconds is about the longest exposure possible, due to electronic detector noise effectively snowing out the image. To achieve the longer exposures I do what many amateur astronomers do. I take multiple 30-second exposures, then 'stack' them using imaging software, thus producing the longer exposure." 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「star trail」の詳細全文を読む スポンサード リンク
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