The Sky-Watcher 1.25 Inch Light Pollution Filter is a contrast-enhancing anodised aluminium, optical glass filter designed to block out the wavelengths of light emitted by mercury-vapour light & other common causes of light pollution, whilst providing higher transmission at critical hydrogen-alpha & hydrogen-beta lines than competing filters. Bright, light-polluted skies appear much darker, & the contrast between object & sky is enhanced significantly. This contrast-enhancement effect is particularly apparent on nebulae. Unlike stars, emission nebulae give off light in a very narrow range of wavelengths.