Chroma SII Filter - 8nm

Isolating the 672.4 nm SII Doublet for High-Signal Nebulae

This filter targets the light emitted by singly ionized sulfur, which appears as a close doublet at 671.6 nm and 673.1 nm. The 8 nm bandpass of the Chroma SII filter is centered at 672.4 nm to capture both of these lines with nearly 100% peak transmission. This signal is a key component in false-color palettes like the SHO (Hubble) palette, where SII data is typically mapped to the red channel to reveal structures in star-forming regions and supernova remnants that are distinct from those seen in Hydrogen-alpha or Oxygen-III.

8nm Bandwidth Performance from f/5 Down to f/2

While designed for systems at f/4.0 and slower, the 8 nm bandpass provides a crucial advantage in faster optical systems where narrower filters suffer significant signal loss. As the focal ratio gets faster, the filter's passband shifts toward blue. The Chroma 8nm SII filter keeps both SII lines inside its half-maximum band down to f/2, though at f/2 the 673.1 nm line sits close to the band's red edge.

• At f/4: The center wavelength shifts to 671.8 nm, close to the centre of the doublet, with 100.0% peak transmission.
• At f/3: The center shifts to 671.2 nm, with 99.8% peak transmission and both lines inside the half-maximum band (667.5 to 675.0 nm).
• At f/2: The passband center is 669.9 nm, with a half-maximum range of 666.0-673.8 nm. The 673.1 nm line is still inside the half-maximum band, though near its red edge; a 3 nm SII filter loses more of the doublet at this focal ratio.

Choosing Your SII Bandwidth: 8nm vs. 5nm and 3nm

Chroma offers SII filters in 3 nm, 5 nm, and 8 nm bandwidths, each representing a different trade-off between signal strength and contrast. The 8 nm version suits imagers for whom signal is the priority.

• Chroma 3nm & 5nm SII: These tighter filters provide higher contrast by rejecting more of the stellar continuum and light pollution background. They excel at pulling fine details out of bright nebulae from light-polluted locations. However, they are more sensitive to the blue shift in fast optical systems.
• Chroma 8nm SII: This filter's wider bandpass captures more photons from the target, making it better for imaging fainter objects, using shorter exposure times, or working with very fast telescopes (f/4 and faster). The trade-off is slightly lower contrast against the sky background compared to its narrower siblings.

Professional Build: 3.0mm Parfocal Substrates and Sputtered Coatings

Chroma's astronomy filters are manufactured in Vermont, USA. The 8nm SII filter is built on a single 3.0 mm thick substrate, which guarantees it is parfocal with other Chroma astronomy filters. This means you can switch between your LRGB, Ha, OIII, and SII filters without needing to refocus, saving valuable imaging time. The durable sputtered hard coatings sit on the front surface with an anti-reflection coating on the rear to prevent ghosting, and the transmitted wavefront is better than 0.25 waves/inch.