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Chroma SII Filter - 3nm
Chroma SII Filter - 3nm

Chroma SII Filter - 3nm

by Chroma
SKU CT-27009-1.25M
$575.00 USD - $1,550.00 USD
$575.00 USD
$575.00 USD - $1,550.00 USD
$575.00 USD
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  • Central Wavelength: 672.4 nm (Sulfur-II)
  • Bandwidth (FWHM): 3 nm
  • Substrate Thickness: 3.0 mm
  • Parfocal: Yes, with other Chroma astronomy filters
  • Recommended Focal Ratio: f/4.0 or slower
  • Transmitted Wavefront: Better than 0.25 waves/inch
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  • Description
  • FAQ
  • Specifications
  • In the Box
  • Warranty

  • Chroma SII Filter - 3nm

    The Chroma SII 3nm Filter is engineered to isolate the faint, deep-red light from ionized sulfur atoms, targeting a central wavelength of 672.4 nm to capture the crucial SII doublet. Its tight 3 nm bandpass delivers high contrast for imaging emission nebulae, built on a 3.0 mm thick substrate that ensures it is parfocal with other Chroma astronomy filters. Designed for systems at f/4.0 or slower, this filter has a transmitted wavefront better than 0.25 waves per inch.

    Isolating the SII Doublet at 672.4 nm for High Contrast

    This filter targets the two adjacent emission lines of singly ionized sulfur (SII) at 671.6 nm and 673.1 nm, a key component in many star-forming regions. By precisely centering its 3 nm passband at 672.4 nm, it captures this signal while cleanly separating it from the dominant Hydrogen-alpha emission at 656.3 nm. This isolation is critical for creating detailed tricolor "Hubble Palette" (SHO) images, where SII data is typically mapped to the red channel to reveal structures that would otherwise be lost in an H-alpha-only image.

    3nm FWHM: The Trade-Off Between Signal and Rejection

    The 3 nm Full-Width at Half-Maximum (FWHM) bandwidth is Chroma's narrowest standard offering, providing the most aggressive rejection of light pollution, moonlight, and continuum light from stars. This results in a darker background and higher-contrast images of faint nebulae. The trade-off for this selectivity is a higher sensitivity to focal ratio shift than the wider 5nm or 8nm filters.

    Focal Ratio Performance: 68.9% Peak Transmission at f/2, 99.8% at f/4

    Interference filters experience a "blue shift" in faster optical systems, where the passband moves to shorter wavelengths. The Chroma 3nm SII filter is designed for peak performance at f/4.0 and slower, where measured peak transmission is 99.8%. At f/3 peak transmission is still 99.7%, but the passband shifts to 669.8-672.8 nm at half maximum, so the 673.1 nm component of the doublet falls outside it. At f/2.5, peak transmission falls to 91.8%, and at a very fast f/2, the blue shift reduces peak transmission to just 68.9% while widening the bandpass to 4.6 nm. For systems near f/3, there is an SII 3nm optimized for f/2.8-f/3.6; for beams faster than f/3, use 5 nm or wider bandpasses.

    Research-Grade Build: 3.0mm Parfocal Substrate and Sputtered Coatings

    Manufactured in Vermont, USA, every Chroma filter is built to exacting standards. The durable, sputtered hard coatings are applied to a single 3.0 mm thick substrate, which makes it parfocal with other filters in the Chroma astronomy line (except the LoGlow), so switching filters needs little or no refocusing. The primary coating is on the front surface and an anti-reflection coating on the rear, which Chroma uses to prevent ghosting. Transmitted wavefront is better than 0.25 waves per inch.

  • What is the Chroma 3nm SII filter used for in astrophotography?

    The Chroma 3nm SII filter is used to capture light from singly ionized sulfur atoms, which emit at 671.6 nm and 673.1 nm. This signal is a key component in many emission nebulae, such as the Soul Nebula (IC 1848). It is essential for creating false-color "Hubble Palette" (SHO) images, where it's typically assigned to the red channel to reveal distinct structural details.

    How will the Chroma 3nm SII filter perform on my f/2 system?

    While usable, it's not optimal. The filter is designed for systems of f/4.0 or slower. On an f/2 system, the converging light cone causes a "blue shift" that moves the filter's passband away from the SII emission lines. This reduces peak transmission to approximately 69% and broadens the bandpass. For beams faster than f/3, use 5 nm or wider bandpasses.

    Should I choose the 3nm, 5nm, or 8nm Chroma SII filter?

    The choice depends on your sky conditions and telescope speed:

    • 3nm: The highest contrast of the three, for light-polluted skies or fine detail. Suits f/4 and slower systems.
    • 5nm: A versatile choice that offers excellent contrast with better signal throughput. It's more tolerant of fast focal ratios than the 3nm; 5 nm or wider is recommended for beams faster than f/3.
    • 8nm: The most tolerant of very fast optical systems, at the cost of more background and lower contrast.

    Is the Chroma 3nm SII filter parfocal with my other filters?

    Yes. The Chroma 3nm SII filter uses a 3.0 mm thick glass substrate, making it parfocal with all other filters in Chroma's astronomy lineup (with the exception of their LoGlow filter). This means you can switch between your LRGB and other narrowband Chroma filters without needing to refocus your camera, saving valuable imaging time.

    What size Chroma SII filter do I need for my astronomy camera?

    The correct size prevents vignetting (darkening of the corners) in your images. As a general guide:

    • 1.25" mounted: Typically suits smaller sensors up to about 16mm in diagonal.
    • 31mm or 36mm unmounted: In a filter wheel, 31mm typically suits sensors up to Four Thirds class (approx. 22mm diagonal) and 36mm suits APS-C class sensors (approx. 28mm diagonal).
    • 50mm round or 50x50mm square: Typically needed for full-frame sensors (43mm diagonal).

    Why are sputtered hard coatings important on the Chroma 3nm SII filter?

    The sputtered hard coatings are extremely durable, withstand humidity changes and extreme temperature fluxes, and remain spectrally accurate in centre wavelength and bandpass.

  • Manufacturer Chroma Technology Corp
    Part Number 27009
    Made In Vermont, USA
    Central Wavelength 672.4 nm (for SII doublet at 671.6 / 673.1 nm)
    Bandwidth (FWHM) 3 nm
    Recommended Focal Ratio f/4.0 or slower
    Substrate Thickness 3.0 mm
    Parfocal Yes, with other Chroma astronomy filters
    Transmitted Wavefront Better than 0.25 waves/inch
    Coatings Durable sputtered hard coatings on a single substrate
    Anti-Reflection Yes, on rear surface
    Available Sizes 1.25" mounted, 2" mounted, 31mm unmounted, 36mm unmounted, 50mm unmounted, 50x50mm square unmounted
    Mounting Threads 1.25": M28.5x0.6 male; 2": M48x0.75 male
    Warranty Chroma lifetime warranty for sputtered catalog filters

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  • Chroma Technology Lifetime Warranty