Astromania 1.25" Narrowband NBPF Hydrogen-a 12nm Filter. Astrophotography under a light-polluted night sky can be difficult. The solution is an H-alpha filter. The Astromania H-alpha cut-off filter works to filter out a large part of this light pollution. This convenient filter only allows 12nm bandwidth of light centered on a wavelength of 656nm to pass through, which means it blocks a large part of the scattered light and also blocks the light from artificial mercury and sodium-vapour lamps. Young stars embedded in the bright red of a nebula - an H-alpha filter is the expert if you want to capture hydrogen nebulae in all their glory. The Astromania H-aplha cut-off filter enhances the contrast between object and sky, allowing you to take good quality images even under urban skies. Metal filter cell threads directly into the 1.25-inch barrel of your telescope eyepiece. Mounted in a nicely anodized housing.