This filter increases the contrast of deep-sky objects like emission nebulae and darkens the background sky by blocking mercury vapor light transmissions and enhancing transmission in the hydrogen beta, doubly ionized oxygen and hydrogen alpha regions of the spectrum. Enhance deep-sky observing in light-polluted skiesBlocks the common wavelengths of light pollution. Improves the view of nebulas, galaxies and clusters. If your going to own a filter, this is the one to have. Darkens the background sky, images "pop" out much better.