The problem of urban illumination is becoming more and more important every year – new houses, new roads, illumination, thoughtless design of urban lighting lead to the fact that the sky turns into a solid orange canvas, and because of the bright background it is impossible to capture even bright nebulae. The situation changed dramatically after the appearance of specialized light pollution suppressor filters for astrophotography. The use of interference coatings made it possible to cut off unnecessary regions of the spectrum — for example, the emission lines of mercury and sodium lamps, and to transmit the regions of interest — emission lines of hydrogen and oxygen.
There are several varieties of light pollution filters — broadband and narrowband. In broadband filters include CLS (City Light Suppression), LPR (Light Pollution Reduction), UHC (Ultra High Contrast), UHC-S. They may be under different brands, but they have one task – to weaken the illumination of city. The UHC bandwidth is narrower in the green area, they are more suitable for heavily exposed areas, but it’s harder to get the corrent white balance. The usual bandwidth in the green region is from 20 to 75 nm. The narrower the filter, the darker the background is. Also, CLS \ UHC filters can pass the red region of the spectrum – this is not important for visual observation, but it is very important for photography, since the emission line of hydrogen nebulae (H-alpha) is in that area.