ZWO 31.75 mm (1.25 in) OIII 7nm Narrowband Astrophotography Filter
The ZWO 31.75 mm (1.25 in) OIII 7nm Filter is an indispensable tool for deep-sky astrophotographers looking to capture the delicate structures of planetary nebulae, supernova remnants, and diffuse emission nebulae. It isolates the crucial 500 nm doubly ionized oxygen (OIII) emission line while completely cutting off surrounding light pollution.
Adding the OIII filter to your imaging workflow allows you to move beyond traditional RGB imaging. Combining it with H-Alpha and SII filters creates stunning false-color images in the iconic Hubble Palette (SHO), or combining it with H-Alpha alone yields highly detailed bicolor (HOO) compositions. With a tight 7 nm bandpass (FWHM 7±0.5 nm) and peak transmission around 90%, this filter yields incredible contrast even under light-polluted urban skies or bright moonlight.
Key Advantages
- 500 nm OIII Peak: Passes green-blue Oxygen-III emissions with ~90% transmission efficiency.
- Hubble & Bicolor Palette Essential: Crucial component for SHO and HOO deep-sky imaging.
- New Coating Standard: Provides significantly higher off-band rejection rates and minimized haloing.
- Infrared Cutoff: Full IR blocking from 700 nm to 1100 nm for sharper stellar detail.
Technical Specifications
| Filter Size | 31.75 mm (1.25 in) mounted |
|---|---|
| Wavelength Peak | 500 nm (OIII line) |
| Bandpass (FWHM) | 7 ± 0.5 nm |
| Light Transmission Rate | ~90% at 500 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 OIII 7nm narrowband filter provides high optical contrast for delicate green-blue oxygen gas structures. Below is the multi-bandpass transmission curve chart alongside the coating comparison showing improved off-band rejection and star halo suppression.
Filter Transmission Curve Graph
New vs. Old OIII Coating Performance
| Brand | ZWO |