The Narrowband StarGuy H Alpha, FWHM 7nm, filter is a deep red long pass filter that enhances the contrast of nebulae emissionsand reduces the effects of light pollution by only passing the hydrogen alpha light. The H Alpha filter is designed to transmit a narrow band of Hydrogen increasing contrast, which is ideal for emission nebulea, like the Horsehead and North American. The filter eliminates halo effects with double sided polishing technique and coatings. 100% tested and inspected by MrStarGuy.