This page assumes you own a telescope and have some experience using it. These selectively block light emitted by Sodium and Mercury vapor lights, and transmit the rest of the spectrum, in particular the wavelenghts emitted by emission nebulae. They are best suited for moderately light polluted skies, but do help on galaxies where light pollution is severe. Narrowband filters transmit primarily on a peak surrounding the Hydrogen Beta and Oxygen III emission lines. They will not work on galaxies, but offer significant enhancement of emission nebulae where light pollution is severe.
Line filters transmit a single emission line.
The O-III filter gives tremendous enhancement of Planatery Nebulae and faint objects such as the Veil Nebula. It even helps in areas where light pollution is negligible, by suppressing the natural airglow. The H-Beta is a specialty filter primarily for the Horsehead Nebula and the California Nebula. Please note - none of these filters will work on open and globular clusters because stars emit a continuous spectrum and are dimmed by these filters. There are 2 methods for coping with dew - passive and active heaters.