Orion 5582 2-Inch Oxygen-III Nebula Eyepiece Filter An Oxygen-III, or O-III, eyepiece filter helps reveal details and improve contrast when viewing nebulas through a telescope Filter transmits close to 90% of the light corresponding to two oxygen-III spectral lines at 496nm and 501nm wavelengths O-III eyepiece filter blocks virtually all other visible wavelengths of light for optimized nebula-viewing performance Made with high quality optical glass in a double-threaded, metal filter cell Recommended for use with telescopes of 8" aperture or greater and for visual use only