FILTERING OUT LIGHT
You can select from a wide range of filters according to your skywatching interests. Filters usually screw into the base of the telescope's eyepiece.
Some filters minimize the effects of light pollution. Known as LPR (light pollution reduction) filters, they block the green and yellow wavelengths emitted by street lights, but transmit the red and blue-green colors of nebulas. While these filters can marginally improve views of star clusters and galaxies, they work best enhancing emission nebulas (ones that produce their own light).
LPR filters come in several varieties. Broadband, or deep-sky, filters transmit the widest range of colors, providing a modest enhancement of a range of deep-sky objects.
Narrowband, or nebula, filters are much more effective on emission nebulas and are the best choice if you are planning to buy just one filter. Line filters, such as Oxygen III and Hydrogen-beta filters, transmit only a single color, providing excellent results on a limited number of nebulas.