Antares ALP/Nebular Filter Light pollution filters, also called nebular filters, are a must when viewing faint objects near city lights. These filters selectively block the major spectral emission lines of both sodium and mercury vapor lamps which are the main two types of urban illumination. These broadband filters also block the natural oxygen line at 557.7nM, while allowing transmission of most of the spectral lines of emission nebula. Comes in a foam-lined plastic case. Filter is threaded to fit on all standard 1.25" eyepiece barrels (M28.5x0.6 thread). Manufacturer part number LPB.