From the Manufacturer's Oxygen-III (O-III) eyepiece filter reveals wondrous details when viewing cloudy nebulas. Filter transmits close to 90% of the light corresponding to 2 oxygen-III spectral lines at 496nm and 501nm for enhanced nebula detail in visual views. Completely blocks virtually all other visible wavelengths of light so views of nebulas are optimized. Eyepiece filter features high quality optical glass in a double-threaded, metal filter cell. Recommended for use with telescopes of 8" aperture or greater for best results and for visual use only