Grain

Like film, DSLR images have grain, though the origin of the grain is different. In film, it's due to irregular clumping of silver halide crystals; in the DSLR, it's due to small differences between pixels. Just as with film, grain is proportional to the ISO setting. Unfortunately, astronomical image processing techniques often increase grain, bringing out irregularity that would never have been visible in a daytime photograph.

Figure 2.1. The Nikon star eater at work. Each photo is the central 300 x 300 pixels of a 30-second exposure of the field of Altair with a Nikon D70s set to ISO 400 and a 50-mm lens at f /4. Note that Mode 3 shows the most stars, but also the most hot pixels. (By the author.)

Mode 3

Long-exposure noise reduction on but interrupted by powering off camera during second exposure

Star Chart

Map of same area, prepared with TheSky version 6, to show which specks are actually stars.

Copyright © 2007 Software Bisque, Inc., www.bisque.com.

Used by permission.

Figure 2.1. The Nikon star eater at work. Each photo is the central 300 x 300 pixels of a 30-second exposure of the field of Altair with a Nikon D70s set to ISO 400 and a 50-mm lens at f /4. Note that Mode 3 shows the most stars, but also the most hot pixels. (By the author.)

Combining multiple images helps reduce grain, especially if the camera was not pointed in exactly the same direction each time. There are also grain-reduction algorithms such as that used in Neat Image (p. 189), an add-on utility that does a remarkably good job of reducing grain without blurring the actual image.

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