| Neutral density eyepiece filters reduce the brightness of the Moon and planets to make it easier to see subtle surface details. Light is transmitted through the filter uniformly over the entire visible spectrum, but reduced in intensity. This makes them excellent for planetary observing, as the bright planetary discs are dimmed to more comfortable observing levels without altering the colors. The filter also helps split binary stars of greatly differing magnitudes by reducing the glare and diffraction around the brighter star of the binary pair. The filters thread into the chrome barrels of virtually all eyepieces available today (exceptions are noted in the individual filter descriptions). They also can be threaded into the nosepiece or color filter wheel carousel of a CCD camera to reduce the image brightness for lunar/planetary imaging. They use polished plane-parallel optical glass to minimize internal reflections and ghosting. They are antireflection coated and mounted in matte black low reflectivity cells with thin knurled grip rings for a more secure hold in cold weather. The filters come in dustproof containers. The filter number is printed on the side of each filter. Each filter (except the Celestron Moon filter) has dual threads to allow you to stack two filters together – to combine one of these neutral density filters with a color filter to simultaneously reduce glare and enhance contrast, for example. See the section on polarizing filters for another method of glare reduction.
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