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.
Passive heaters are heated strips which wrap around a telescope's objective, eyepiece, and finder.
They maintain the temperature of the optics a few degrees above ambient. They must be turned on as soon as the telescope is set up - they do not have enough heat to evaporate dew once it forms. They are usually about 150 watts, and are best run off car batteries as they draw a lot of current. Active heaters are used to 'zap' optics once dew begins to form. Dew guns are the only resort on nights which are so humid your telescope is literally dripping.