An H-beta filter exploits the properties of so-called emission nebulae. Some objects glow in certain colours, known as emission lines. The emission lines are linked to specific chemical elements – in this case, hydrogen. This high-quality line filter blocks all other colours and thus almost all artificial light, allowing only the emission line of hydrogen at 486 nm to pass through. The effect is amazing: suddenly, nebulae become visible in places that appear completely empty without a filter. For example, with a slightly brightened sky, the Horsehead Nebula is practically invisible with a 300 mm telescope. When using this filter, the nebula can be seen without any problems.
