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Chroma OIII Filter - 8nm
Chroma OIII Filter - 8nm

Chroma OIII Filter - 8nm

by Chroma
SKU CT-27008-1.25M
CA$385.00 - CA$1,162.00
CA$385.00
CA$385.00 - CA$1,162.00
CA$385.00
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  • Isolates the 500.7nm OIII emission line
  • 8nm bandpass for high signal throughput
  • Designed for f/4.0 or slower; measured 99.7% peak transmission at f/2
  • 3.0mm thick single substrate, parfocal with other Chroma astronomy filters (except LoGlow)
  • Durable sputtered hard coatings, anti-reflection coated rear surface
  • Transmitted wavefront better than 0.25 waves/inch
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  • Description
  • FAQ
  • Specifications
  • In the Box
  • Warranty

  • Chroma OIII Filter - 8nm Bandpass

    The Chroma OIII 8nm Filter isolates the critical 500.7nm emission line from doubly ionized oxygen, capturing high-contrast signal from planetary nebulae and supernova remnants. Its 8nm bandpass is designed for higher signal throughput, achieving measured peak transmission of 99.8% at f/4 while remaining parfocal with all other 3.0mm Chroma astronomy filters. Built on a single, non-laminated substrate with a transmitted wavefront better than 0.25 waves/inch, this filter also has a rear anti-reflection coating to prevent ghosting.

    8nm Bandpass for High-Signal OIII Imaging

    The 8nm full width at half maximum (FWHM) bandwidth is an ideal balance for capturing the faint, ethereal structures of emission nebulae. This passband is wide enough to collect significant signal quickly, reducing exposure times compared to narrower filters, making it a powerful tool for imaging targets like the Veil Nebula (NGC 6960) or the Dumbbell Nebula (M27). By blocking surrounding light pollution and continuum emission, the filter dramatically increases the contrast of these OIII-rich objects against the sky background.

    99.7% Peak Transmission at f/2

    While designed for systems of f/4.0 and slower, measured data shows the 8nm OIII filter holds its transmission in fast astrographs. All interference filters experience a "blue shift" in faster focal ratios, where the passband moves to shorter wavelengths. The 8nm bandwidth provides generous headroom, keeping the 500.7nm OIII line well within the passband even under aggressive focal reduction. Measured data shows the filter's center wavelength is 500.6nm at f/4 and 499.4nm at f/2, where peak transmission is 99.7%.

    3.0mm Parfocal Design with Sputtered Coatings

    Chroma's astronomy filters are manufactured in Vermont, USA. The OIII 8nm is built on a single 3.0mm thick glass substrate with a transmitted wavefront better than 0.25 waves/inch. This thickness standardizes the optical path, making the filter parfocal with other Chroma LRGB and narrowband filters, eliminating the need to refocus when switching between them. Durable sputtered hard coatings sit on the front, with an anti-reflection coating on the rear surface to prevent ghosting, and the filter is covered by Chroma's lifetime warranty for sputtered catalog filters.

    8nm vs. 5nm/3nm: Balancing Signal and Contrast

    Choosing a bandwidth is about managing trade-offs for your specific imaging goals and equipment. The 8nm filter prioritizes signal strength and focal ratio tolerance, making it a good fit for:

    • Faster Optical Systems (f/5 or faster): The wider band is more forgiving of the blue shift inherent in fast optics, keeping the OIII line inside the passband (measured down to f/2).
    • Shorter Total Integration Times: Captures more photons per minute, ideal for portable setups or limited imaging windows.

    In contrast, Chroma's 3nm OIII is designed for maximum contrast and background suppression, ideal under light-polluted skies or bright moon phases, and the 5nm balances strong contrast with higher signal throughput. However, they demand more precise focal ratio matching and longer integration times to achieve a comparable signal level.

  • Why choose the 8nm Chroma OIII over a 3nm or 5nm version?

    The 8nm Chroma OIII filter gives higher signal throughput and more tolerance of fast optical systems than the 3nm and 5nm versions. Its wider bandpass is more tolerant of the blue shift that occurs in fast scopes and allows you to capture more light in shorter exposures. Tighter 3nm or 5nm filters offer higher contrast but require longer exposures and are less forgiving in fast optics.

    How does the 8nm Chroma OIII perform in a fast f/2 system?

    At f/2 the passband shifts blue, but its 8nm width keeps the 500.7nm OIII line within its window. Measured data shows the filter's center at 499.4nm at f/2 with 99.7% peak transmission. For beams faster than f/3, use 5nm or wider bands.

    What deep-sky objects are best for the Chroma OIII 8nm filter?

    This filter is ideal for emission nebulae that are strong in doubly ionized oxygen. Prime targets include:

    • Planetary Nebulae: The Dumbbell Nebula (M27), the Ring Nebula (M57), and the Helix Nebula (NGC 7293).
    • Supernova Remnants: The entire Veil Nebula complex (NGC 6960, 6992, and 6979) and the Crab Nebula (M1).
    • Large Emission Regions: Portions of the Eagle Nebula (M16) and Lagoon Nebula (M8).

    Is the 8nm OIII filter parfocal with other Chroma filters?

    Yes. All Chroma astronomy filters (with the exception of the LoGlow filter) are manufactured on a 3.0mm thick substrate. This ensures they are parfocal, meaning you can swap between your Chroma LRGB and other narrowband filters without having to refocus your telescope, saving valuable imaging time.

    What sensor size is the 2" mounted Chroma OIII 8nm filter suitable for?

    The 2" mounted filter (threaded M48x0.75) typically suits sensors up to APS-C class; on a full-frame sensor (36x24mm) it may vignette, so 50 mm unmounted filters are the usual choice there. Using a filter that is too small for the sensor can cause vignetting, or darkening, at the corners of your image. The 1.25" filter typically suits smaller sensors, around 16mm diagonal or less.

    Does the blue shift at fast focal ratios make the 8nm Chroma OIII unusable?

    No, quite the opposite. The 8nm bandwidth is specifically what makes this filter so robust against blue shift. While a very tight 3nm filter might shift enough to move the 500.7nm OIII line to the edge of its passband (reducing signal), the 8nm filter provides ample room, with 99.7% measured peak transmission at f/2.

  • Manufacturer Chroma Technology Corp
    Part Number 27008
    Made In Vermont, USA
    Central Wavelength 500.7 nm (OIII Line)
    Bandwidth (FWHM) 8 nm
    Peak Transmission (at f/4) 99.8%
    Peak Transmission (at f/2) 99.7%
    Designed Focal Ratio f/4.0 or slower
    Glass Substrate Single 3.0 mm thick piece
    Parfocal Yes, with other 3.0mm Chroma astronomy filters
    Transmitted Wavefront Better than 0.25 waves/inch
    Coatings Durable sputtered hard coatings, AR-coated rear surface
    Sizes Offered 1.25" mounted, 2" mounted, 31mm unmounted, 36mm unmounted, 50mm unmounted, 50x50mm unmounted
    Cell Threads 1.25" (M28.5x0.6), 2" (M48x0.75)
    Warranty Chroma Lifetime Warranty
    Glass Thickness 3 mm

    • Chroma OIII Filter - 8nm

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  • Chroma Technology Lifetime Warranty