Nebular Filters
Nebular filters pass selected portions of the visual spectrum and serve to increase the contrast and detail visible in emission nebulae. They are sometimes referred to as light pollution filters, which is more than a little bit misleading. What these filters do is to selectively allow the light being emitted by a nebula to pass while blocking other parts of the visible spectrum. This blocks both nearby starlight and some light coming from man made sources. The most universally useful filter of this type is known as a narrowband or ultra high contrast (UHC) filter. These filters are ideal for star forming regions, such as the Orion or Lagoon Nebula. Oxygen-III filters rank as the second most useful of the nebular filter types and are best applied to supernovae remnants that are rich in oxygen.