This 48mm narrowband filter has a 280 Ångstrom passband for visually observing emission nebulas from heavily light-polluted areas using medium and long focal length 2” eyepieces. The filter has excellent transmission (generally well in excess of 95-96%) at all 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. It has very steep passband cutoff slopes, to provide higher contrast than conventional narrowband filters that have more gradual transition slopes. 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. It has twice the contrast of the Thousand Oaks broadband filter, making it ideal for faint nebulas such as the Helix, Rosette, Veil, and North American. Brighter nebulas (such as the Omega, Dumbbell, Lagoon, etc.) stand out well under city lights. All coatings are sandwiched between two layers of glass to protect them against accidents and cleaning that’s a little too aggressive.
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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