I have accumulated a sufficient number of different light pollution filters.I decided to conduct their comparative testing both with a spectroscope and in the illuminated city night sky.To take the spectra, we used an inexpensive spectroscope with aliexpress, Canon 550Da camera (a blue filter was removed), an Industar 50-2 lens, and the Sun as a light source.The night test was carried out in the center of Anapa, and the illumination from the Moon also prevented.The results are below.
Spectrums of astronomical light pollution filters.
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.
Протестировал по небу бюджетный широкополосный UHC фильтр Everest с Aliexpress. Данный фильтр также может попадаться под маркировками Solomark, T-optics. Главная задача UHC фильтра – выделение деталей в газовых туманностях, в том числе и планетарных. Также фильтр позволяет ослабить световое загрязнение, притемняя фон и выделяя туманность. По звездным скоплениям и галактикам UHC фильтр лучше не использовать. Резьба на фильтре – стандартная под 1.25″ (М28.5).