The Thousand Oaks Optical Narrowband Light Pollution Reduction Filter is mainly designed for viewing emission nebulae from low to moderate light polluted areas. Often, its use in severely light polluted areas may be your only hope of observing nebular objects at all. The filter allows only the spectral emission lines of Oxygen III (OIII) and Hydrogen Beta (H-Beta) to pass through, as well as the wavelengths between the H-Beta and OIII lines. Such filters are often marketed as "nebula" filters.
The effect of narrowband filters on emission nebulae can be breathtaking with faint nebular objects becoming easier to see. The filter darkens the background sky glow substantially without degrading the nebulae. Prominent nebulae visible without filters can gain significant detail and contrast with a narrowband filter. Under true dark sky conditions the contrast and detail improvements are impressive.
Narrowband filters reduce the brightness of most star clusters, reflection nebulae, and galaxies. Photographic use of these narrowband filters is not recommended.
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