| Chromatic aberration (spurious color) is an optical defect in telescopes that results in an annoying faint blue-violet halo of out-of-focus light around the Moon, planets, and bright stars. Whenever light passes through one material to another, different wavelengths of light (different colors) are bent (or refracted) by different amounts. This is a problem that plagues refractor-type telescopes, since light passes through both air and glass to form an image. Bright astronomical objects emit a spectrum comprised of many different wavelengths of light, so each wavelength will be bent by a slightly different amount when passing through a lens. This results in each color of light reaching precise focus at a slightly different point. The most common visual effect of this is a purplish halo around the object being viewed. This halo effect is most visible in low cost achromatic refractor telescopes and can mask lunar and planetary details. These chromatic aberration filters greatly reduce this spurious color fringing of blue-violet light, without appreciably changing the color of the object being observed. Viewing pleasure is enhanced, not only because false color is reduced but also because focusing is made easier. Since the filter reduces the amount of unfocused light reaching the eye, finding the focus "sweet spot" is made easier. Chromatic aberration filters should only be used when observing bright objects, such as the Moon, the bright planets (Venus, Mars, Jupiter, and Saturn) or bright stars. The filters will not have any effect when observing faint objects, such as nebulas and galaxies, because they do not emit enough light to cause visible chromatic aberration.>
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