These OVL telescope filters in their black anodized aluminum frame are designed to filter out the interfering light from mercury vapor lamps and other light pollution, while allowing critical H-alpha and H-beta wavelengths to pass better than competing filters. Bright, light-polluted skies appear much darker and the contrast between sky and object is visibly increased. This contrast enhancement effect is particularly important in fog. Unlike stars, nebulae only emit light in a very limited range of wavelengths. These filters allow the maximum transmission of the important wavelengths in H-alpha, H-beta and the two lines of ionized oxygen, these are the most important wavelength ranges in nebulae. Observations of galaxies and star clusters also benefit from this filter, although not as much. Furthermore, the contrast of reddish details on planets is improved. This filter works with all telescope sizes, but is particularly well suited for telescopes up to 114 mm in diameter, where other filters remove too much light. Scope of delivery: Filter 1.25″ storage box! Power supply and batteries are not included!