| Unlike stars, emission nebulas give off light in a very narrow range of wavelengths. You can’t turn up the luminosity on faint nebulas to make them easier to see, but you can increase the contrast between them and the sky to reveal more of their wondrous detail. This Orion 1.25" Oxygen III line filter has a narrow 150 Ångstrom FWHM (Full-Width at Half-Maximum transmission) passband with excellent transmission in the 90%-92% range of the most common green nebula emission lines of doubly ionized oxygen at 4959Å and 5007Å, but virtually complete blockage of all other visible light, including light pollution. Its very narrow passband gives the high contrast you need for observing famous nebulas like the Swan, Ring, Dumbbell, Helix, and Eagle from heavily light-polluted city locations. It is excellent for visually observing very faint planetary and gaseous nebulas (such as M76 – the Little Dumbbell – and Barnard’s Loop in Orion) from mildly to moderately light-polluted sites. These objects are normally invisible without a filter. The narrow passband and high Oxygen III transmission combine to make emission and planetary nebulas stand out crisply against the blackness of space, from dark sky and light-polluted observing sites alike. It is not designed for astrophotography or for observing reflection nebulas. The filter is best with medium to large aperture telescopes (8" and larger). The filter comes in a protective hard plastic case. An individually-measured light transmission curve is supplied with each filter. Enter your telescope focal ratio in the formula box to the right to see what focal length eyepieces will work best with this filter.
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