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This large area of emission nebulosity surrounds Sadr, Gamma Cygni, the brightest star in this photo in the center of the frame, and the heart of the constellation of Cygnus the Swan. Numerous star clusters can also be found embedded in the nebulosity. The two sections of bright nebulosity to the left of Sadr are called the Butterfly Nebula, and have the catalog designation of IC 1318. The large area of dark nebulosity that separates the two halves of the Butterfly is LDN 889. The large patch of bright nebulosity at the top of the frame is designated as IC 1318a. The Crescent Nebula, NGC 6888, is at the lower right. This image was shot from my driveway in light-polluted suburban Philadelphia with a nearly full moon up. The special narrowband hydrogen-alpha filter that was used made this possible. In the mouse-over comparison below, we can see the tremendous difference that the filter makes.
Because the narrowband 6nm hydrogen-alpha filter only passes light in a narrow window just 6 nanometers wide centered on 656.3 nanometers, the wavelength of the red hydrogen-alpha emission, it blocks out most of the other light in the sky from the Moon and from light pollution. This wouldn't work on a continuous spectrum object such as a reflection nebula, or a galaxy. The light from those objects is spread out across the entire spectrum. Because this light is red, it is only recorded in the red channel of a color image shot with a DSLR. Because we don't have any data in the green and blue channels, we can't produce a color image, therefore hydrogen-alpha narrowband images are monochrome. Because most stock DSLR cameras have a long-wavelength filter that filters out most of the hydrogen-alpha wavelength, it is advisable to use a modified DSLR with a narrowband hydrogen-alpha filter in a DSLR. Exposure Data
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