| Unlike stars, emission nebulas give off light in a very narrow range of wavelengths. This Orion UltraBlock 2" ultra high contrast light pollution filter has a narrow 270 Ångstrom FWHM (Full-Width at Half-Maximum transmission) passband that allows maximum transmission of the light of ionized gasses that is most commonly emitted by nebulas, while blocking the unwanted glare of city lights. It improves the contrast of emission nebulas viewed from badly light-polluted areas, sometimes making the difference in being able to see them at all from a city site. The filter has excellent transmission in the +/-90% range at the visually important emission lines of hydrogen-beta at 4861Å and doubly ionized oxygen at 4959Å and 5007Å. This combines with the narrow passband to make emission nebulas stand out briskly against a dark sky. As the filter also eliminates the natural airglow of ionized oxygen and nitrogen in our own atmosphere, it can also be useful in hunting down and enhancing very faint nebulas from a dark sky observing site. It is ideal for faint nebulas such as the Helix, Rosette, Veil, and North American from such sites. Brighter nebulas (such as the Omega, Dumbbell, Lagoon, etc.) stand out well under city lights. It is not designed for astrophotography or for observing reflection nebulas. 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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