| This 1.25" Astro-Tech Oxygen III line filter has a very narrow 110 Ångstrom FWHM (Full-Width at Half-Maximum transmission) passband that is centered on the nebula emission lines of doubly ionized oxygen at 4959Å and 5007Å where most nebulas radiate the strongest.
Its very narrow passband is exceptionally efficient at blocking city light pollution, Moonglow, and natural airglow. It gives you two to three times the contrast of a narrowband (ultra-high contrast) filter when observing brighter nebulas like M17 and M27 from heavily light-polluted city locations. It very often gives you your only opportunity to observe this type of object when the Moon is up (although observing when the Moon is new is always the best way to completely eliminate pesky Moonglow). Very faint planetary and gaseous nebulas (such as M76 and Barnard’s Loop in Orion), that are normally invisible from mildly to moderately light-polluted sites, are often easy targets through this very high contrast filter.
The Astro-Tech Oxygen III filter typically has a peak Oxygen III line transmission of 94% to 95% region. This high transmission combines with the very narrow passband to make emission nebulas stand out crisply against the blackness of space. It can also be useful in hunting down and enhancing very faint nebulas from a dark sky observing site. It is not designed for astrophotography or for observing reflection nebulas or galaxies. This filter is best with medium to large aperture scopes (8" and larger).
There is some minor leakage in the deep ultra-violet portion of the visible spectrum that can be seen in the transmission graph feature image below, but the transmission of the mercury light pollution lines near 4050Å and 4370Å is essentially zero. Only the youngest observers, those whose vision still extends into the deep ultra-violet, might see a trace of chromatic aberration if this filter is used on a low quality achromatic refractor. Older observers, whose ultra-violet vision started to degrade when they passed 25 years, are unlikely to see any trace of the leakage.
A comparison of the transmission graphs in the feature image below shows how well this Astro-Tech filter’s performance compares with that of the best-known Oxygen III filter, but at only a fraction the price of the best-known filter. The Astro-Tech Oxygen III line filter is another good example of the Astro-Tech philosophy of “more performance for less money." The Astro-Tech filter comes packed in a foam-lined snap-lid plastic storage case.
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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