Chroma SII Filter - 5nm

The Chroma 5nm SII filter isolates light from singly ionized sulfur, found in many emission nebulae. It is a critical component for creating false-color "Hubble Palette" (SHO) images, where SII data is typically assigned to the red channel, H-alpha to green, and OIII to blue. This technique reveals structure and differentiation in gas clouds that would otherwise appear uniformly red in a standard color image.

Chroma designs this filter for f/4.0 or slower, and recommends 5 nm or wider filters for beams faster than f/3. Measured data shows 99.1% peak transmission at f/3 and 94.0% at f/2, with the passband centre shifting to 671.4 nm at f/3 and 670.2 nm at f/2. At f/2 the half-maximum passband (667.7 to 672.7 nm) no longer covers the 673.1 nm line of the doublet.

The choice depends on your sky conditions and telescope speed.

• The 5nm filter offers a balance of contrast and signal throughput. It is more forgiving with fast focal ratios (f/4 and below).
• A 3nm filter provides higher contrast against skyglow, useful from light-polluted locations. However, it is more sensitive to blue shift in fast scopes.

No. All Chroma astronomy filters (with the exception of the LoGlow filter) are specified to be parfocal. They are all built on 3.0 mm thick substrates, ensuring that your focus point remains constant when you switch between filters in a filter wheel, saving valuable imaging time.

The astronomy filters are rated research grade optical quality: durable sputtered hard coatings on a single 3.0 mm substrate with no laminations, the primary coating on the front surface and an anti-reflection coating on the rear to prevent ghosting, and a transmitted wavefront better than 0.25 waves/inch.

Yes, absolutely. The Soul Nebula is a classic emission nebula rich in ionized gases, including sulfur. Using the Chroma 5nm SII filter will allow you to capture the specific structure of the sulfur emissions within the nebula, which is key to creating a dynamic and colorful SHO (Hubble Palette) image of this popular target.

The correct size prevents vignetting (darkening of the corners) in your image.

• 1.25" Mounted: Suitable for small sensors, typically up to a 12mm diagonal.
• 31mm Unmounted / 36mm Unmounted: 31mm typically suits sensors up to about the Four Thirds class; 36mm typically suits APS-C class sensors.
• 2" Mounted / 50mm Round or Square: Typically needed for full-frame sensors (36x24mm) to ensure the entire sensor is illuminated without vignetting.