These OVL telescopic filters in their black anodized aluminum socket are designed to filter out the disturbing light from mercury vapor lamps and other light pollution, while allowing critical H-alpha and H-beta wavelengths to pass through better than competing filters. Bright, light-polluted skies appear much darker and the contrast between the sky and the object is visibly enhanced. This contrast enhancement effect is especially important in foggy conditions. Unlike stars, nebulae emit light only in a very limited wavelength range. These filters allow the maximum transmission of the important wavelengths in H-alpha, H-beta and the two lines of inonized oxygen, which are the most important wavelength ranges for nebulae. Observations of galaxies and star clusters also benefit from this filter, although not as much. Furthermore, the contrast is improved for reddish details on planets. This filter works with all telescope sizes, but is particularly suitable for telescopes up to 114 mm in diameter, where other filters take away too much light. Scope of delivery: Filter 2″ storage box ! Power supply and batteries are not included!