| This Orion Extra Narrowband set of three 1.25" imaging filters lets you take high-contrast CCD images of emission nebulas in full moonlight or in extreme light pollution – at a substantial savings over the cost of buying the filters individually. While they can be used individually for monochrome CCD imaging, these high-performance Orion filters are designed primarily for “false" tri-color deep-sky imaging of emission nebulas using monochrome CCD imagers, such as the Orion StarShoot Deep Space Monochrome Imager II, and a color filter wheel.
The Orion Extra Narrowband set consists of a 1.25" Hydrogen-Alpha filter, a 1.25" Oxygen III filter, and a 1.25" Sulfur II filter. Click on the individual filter links for more complete information about each filter. The filters have very narrow 7nm FWHM (full width at half-maximum) passbands that transmits 90 percent of the light emitted by nebulas at their respective emission lines. The filters block out all other light, including moonlight and artificial light pollution (city streetlights and parking lot lights). They also block infrared (IR) light. The result is a dramatic boost in contrast, revealing subtle nebular emission details that are usually washed out by contrast-killing broadband light.
The Orion Extra Narrowband filters are made from precision-polished high-grade optical glass with multiple-layer vacuum-deposited dichroic interference coatings and antireflection coatings. The filters are supplied in 1.25" cells that thread into the 1.25" nosepiece of a CCD camera for monochrome imaging or into the Orion Multiple Filter Wheel, as well as any color filter wheel that accepts standard 1.25" filters. The filters are supplied in foam-fitted hard plastic protective cases. The Orion Extra Narrowband filters are parfocal, eliminating the need to refocus when switching between them during tri-color imaging.
In addition to monochrome imaging of specific emission lines, the filters can be used for capturing luminance data for LRGB color imaging of emission nebulas with monochrome CCD imagers. When the specific monochromatic emission line exposure is combined with the broader exposures taken through the conventional red, green, and blue filters, the resulting color image often teems with details that are “missing in action" in standard tri-color shots.
Despite their use in monochrome imaging, and the extra details to be gained in true color imaging by combining an individual Extra Narrowband filter exposure with the normal RGB exposures as mentioned above, many people use a combination of only these H-alpha, Sulfur II, and Oxygen III filters for “false" tri-color CCD imaging, rather than the traditional RGB filters. Tri-color images taken with extra-narrowband filters typically look much different from those acquired using standard “true color" (red, green, blue filter) methods.
Rather than rendering an image in its natural color, “false" tri-color imaging with extra-narrowband filters provide a visual representation of the elemental makeup of a nebula. The different colors highlight the areas of the nebula where the light derives predominantly from either Sulfur II, Oxygen III, or Hydrogen-alpha emissions. This is the approach used by astronomers for many of the nebula pictures taken with the Hubble Space Telescope. It’s a technique that allows amateur astrophotographers to emulate the amazing false-color Hubble Space Telescope images, such as the famous “Pillars of Creation." The extra narrowband “false color" images, while beautiful in their own right, also reveal much about the chemical composition of the nebula and its various features.
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