The Explore Scientific 2" S-II Nebula Filter (12nm) filter uses the property of the so-called emission mist. These objects glow in certain colors, the so-called emission lines. The emission lines are linked to certain chemical elements, in this case ionized sulfur. This filter blocks almost all artificial light to improve viewing of nebulae, passing only the emission lines of the the ionized sulfur at 672nm. The effect is amazing: Suddenly fog becomes visible in the photos in places that appear completely empty without a filter. The Explore Scientific mist filters are supplied with an individual test report, so you can be sure that you are receiving high-quality filters.