Photography has two main advantages over visual observing—it keeps a permanent record of what is seen, and long exposures can build up images of objects far fainter than those visible to the naked eye. Astrophotography is a popular occupation for many amateurs.

capturing images

Digital imaging has almost completely supplanted film for astrophotography, as it has with normal photography. Digital cameras contain light-sensitive silicon chips known as charge-coupled devices (CCDs). CCDs have many advantages over film, including much greater sensitivity. Most astronomical objects are so faint that a long exposure is needed to see them. In a few seconds, a CCD can capture an image that might require an exposure of many minutes on film. When the exposure is finished, the image is simply read off the chip into a computer.

The CCD camera is attached to the telescope in place of the eyepiece. For short exposures, the body of a digital camera with a removable lens can be used, but for longer exposures a special CCD designed for astrophotography removing the eyepiece

Eyepieces push into place and are held by small screws. To remove the eyepiece, simply loosen the screws.

is necessary. These are cooled to reduce electronic noise in the chip. several exposures of the same object can be added together to bring out faint features.

Color pictures are created by taking three separate exposures through red, green, and blue filters and combining them in the computer. Further processing can be done to adjust the color balance, brightness, and contrast, and to sharpen detail. The observer need not even be at the telescope to make the exposures, but can sit

using filters to improve images

Special filters are available that reduce the effects of light pollution. These light pollution reduction (LPR) filters make the sky darker so that faint nebulae and galaxies stand out better. Others, called nebula filters, allow through only the specific wavelengths of light emitted most strongly by nebulae. Ultra high contrast (UHC) filters transmit

8 the green light emitted by hydrogen and oxygen. Most restrictive of all are the so-

called OIII filters, which block all but the lines of ionized oxygen that are emitted strongly by planetary nebulae, but these are for specialized use only. All such filters screw into the barrel of the eyepiece.

use a solar filter for obsen/ing the Sun use a solar filter for obsen/ing the Sun

colored filters

Advanced observers use colored filters to enhance details on the planets. For example, yellow and orange filters can emphasize dark markings on Mars, while light green and blue filters can bring out detail in the clouds of Jupiter and Saturn.

solar filters

Solar filters consist of thin plastic, or sometimes glass, with a metallic coating that reduces the incoming light and heat from the Sun to safe levels. Such filters fit across the entire telescope tube, either refractor or reflector, and with one of these in place you can look directly at the Sun to see sunspots and other features. Never use glass Sun filters that cover the eyepiece—these can crack suddenly under the concentrated light and heat, with disastrous consequences for your eyes.

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