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J For the amateur astronomer, the development of the affordable CCD camera

O 0£ has been as important to the advancement of our science as big telescopes and

Z O high-speed photographic films were to astronomers of ages past. According to one professional astronomer whose specialty is instrumentation, the CCD camera is allowing amateurs to do scientific work that is as good or better than the work professionals were doing as recently as the 1970s.

There are numerous books available today on the subject of CCD imaging, written by real experts that discuss CCD imaging in great detail. So many books have been written by such smart people, that I would refer you to them and their knowledge of CCD imaging. One of the best books is one in the Series of Patrick Moore books, edited by David Ratledge titled The Art and Science of CCD Astronomy. Several talented CCD imagers contributed to the book and I highly recommend it. You can learn more from them than I can teach you in the few pages available here. However, it will serve us well to discuss the advantages of CCD imaging over film and the results that can be attained.

Like film, a CCD image is an absolutely objective record of the physical appearance of Jupiter and the placement of its features. The placement of its features; this is important! A CCD camera is so sensitive that it can capture an image in a fraction of the exposure time required for film, effectively freezing the image. Consequently, a CCD image suffers far less from the effects of "seeing" than film does. At best, film captures 3-5% of the light that lands on it. By contrast, a CCD chip captures 30-50% of the light that strikes it, a tremendous gain in light gathering ability [513].

The term "CCD" stands for charge coupled device. A CCD is a silicon chip. Originally developed as a storage device, it was discovered that a silicon chip is sensitive to light. And whereas a silicon chip is normally encased in a black plastic case to keep light out, a CCD chip is made with a window opening on its top to let light in. Silicon is sensitive to the visible and near infrared part of the spectrum. It is sensitive in the sense that it will convert incident light (photons) into an electric charge (electrons). The active light-exposure part of the CCD is divided into photosites or pixels in a matrix of rows and columns. Each photosite converts light (photons) into electrons and stores them until the end of the exposure. The number of photons produced is proportional to the light intensity. These photosites actually count the electrons that are produced when photons strike them. The chip takes the place of film in the camera. The rest of the CCD camera comprises electronic devices to record and digitize the signal, plus a cooling system to keep it cool [514]. The electronic video camera or still camera that you have at home contains a small CCD chip, maybe just 1/8-inch square. The Hubble Space Telescope uses one that is quite sophisticated and large. A local amateur observatory here owns an astronomy CCD camera with a chip 1-in. square. As you would expect, cameras with larger chips are more expensive. So far, most astronomy CCD cameras do not have chips as big as 35 mm film negatives, but those size chips are now available. Consequently, they cannot capture a field of view as large as film. However, for planetary imaging, we don't care. A small chip can work well for us.

Astronomy CCD cameras do not have a camera lens. Rather, the camera is fitted with a barrel similar to an eyepiece and is placed into the eyepiece holder of the telescope, much like a 35mm camera body is attached to a telescope for prime focus or eyepiece projection photography. Like a film camera, a CCD camera can be used for long exposures or short ones. Thus, CCD cameras are suited for faint, deep sky work as well as planetary imaging.

As books on CCD cameras will point out, it is important to match pixel size with the image scale produced by your telescope. Once the image is taken, there is then the problem of processing the image in your computer to bring out detail in the finished image. Don Parker points out that, even though the raw CCD image requires further processing, computer work has many advantages over the long hours astrophotogra-phers used to spend in the darkroom with funny lights and chemicals!

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