Orion 5583 1.25-Inch Hydrogen-Beta Eyepiece Filter Orion's Hydrogen-Beta eyepiece filter makes it possible to view the elusive Horsehead, California, and Cocoon Nebulas in dark skies Selective band-pass filter transmits only the hydrogen-beta light these faint nebulas emit Filter allows you to observe targets that are considered trophies of visual astronomy Best used with a moderate to large aperture telescope and clear, dark skies Metal filter cell threaded to fit 1.25" eyepieces and threaded diagonal barrels