ZWO 31.75 mm (1.25 in) SII 7nm Narrowband Astrophotography Filter
The ZWO 31.75 mm (1.25 in) SII 7nm Filter is specifically designed for deep-sky narrowband astrophotography. It isolates the faint 672 nm Sulfur-II emission line, allowing astrophotographers to capture complex ionized gas structures in emission nebulae, planetary nebulae, and supernova remnants.
When combined with H-Alpha and OIII narrowband filters (forming the standard SHO narrowband set), this filter enables the creation of iconic "Hubble Palette" false-color images, such as the famous "Pillars of Creation" in the Eagle Nebula (M16). Featuring a tight 7 nm bandpass (FWHM 7±0.5 nm) and ~90% peak light transmission, it effectively suppresses light pollution from mercury and sodium vapor street lamps, as well as bright moonlight, letting you capture stunning deep-sky detail even during full moon nights.
Key Advantages
- 672 nm SII Bandpass: Selectively passes ionized sulfur emissions with ~90% transmission efficiency.
- Hubble Palette Ready: Essential channel for SHO tricolor composite astrophotography.
- Refined Base Glass: New optical substrate minimizes star halos and internal reflections.
- Infrared Cutoff: Blocks IR spectrum from 700 nm to 1100 nm for sharper star profiles.
Technical Specifications
| Filter Size | 31.75 mm (1.25 in) mounted |
|---|---|
| Wavelength Peak | 672 nm (S-II line) |
| Bandpass (FWHM) | 7 ± 0.5 nm |
| Light Transmission Rate | ~90% at 672 nm |
| Glass Thickness | 1.85 mm to 2.0 mm (0.073 in to 0.079 in) |
| Cell Height | 5.5 mm (0.22 in) cell + 2.5 mm to 3.5 mm thread |
| Thread Size | M28.5 x 0.6 mm Male Thread |
| Surface Quality | Optically fine-polished to 1/4 wavefront accuracy |
| Off-Band Rejection | <0.1% transmission (Optical Density OD3) |
| Infrared Blocking | 700 nm to 1100 nm cutoff range |
Transmission Curve & Performance Testing
The ZWO SII 7nm narrowband filter isolates faint sulfur emissions from deep-sky nebulae. Below is the multi-bandpass transmission curve chart along with the optical comparison showing reduced haloing on bright stars using the updated glass substrate.
Filter Transmission Curve Graph
New vs. Old SII Glass Halo Comparison
| Brand | ZWO |