ZWO 31.75 mm (1.25 in) H-Alpha 7nm Narrowband Astrophotography Filter
The ZWO 31.75 mm (1.25 in) H-Alpha 7nm Filter is an essential tool for deep-sky astrophotography, designed to isolate the critical 656 nm hydrogen-alpha emission spectrum while effectively blocking unwanted light pollution. Built around high-performance optical glass, this filter allows astrophotographers to capture crisp, high-contrast nebula structures even under heavily light-polluted urban skies or strong moonlight.
With a tight 7 nm bandpass (FWHM 7±0.5 nm) and peak light transmission reaching up to 90%, it selectively permits emission light from gas nebulae while cutting off artificial mercury and sodium vapor street lamps, as well as natural atmospheric skyglow. The revised coating technology reduces optical haloing around bright stars, ensuring pristine signal quality for CCD and CMOS astronomy cameras.
Key Advantages
- 656 nm Emission Band: Passes H-Alpha hydrogen emissions with ~80-90% light efficiency.
- Anti-Halo Coating: Improved substrate glass reduces star reflections and optical halos.
- Infrared Cutoff: Complete blocking of IR wavelengths (700 mm to 1100 nm).
- Standard Threading: Fits 31.75 mm (1.25 in) filter holders with M28.5 x 0.6 mm male thread.
Technical Specifications
| Filter Size | 31.75 mm (1.25 in) mounted |
|---|---|
| Wavelength Peak | 656 nm (H-Alpha line) |
| Bandpass (FWHM) | 7 ± 0.5 nm |
| Light Transmission Rate | ~80% to 90% at 656 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 mm to 1100 nm cutoff range |
Transmission Curve & Performance Testing
The ZWO H-Alpha 7nm narrowband filter isolates spectral emissions from faint emission nebulae, planetary nebulae, and supernova remnants. Below is the optical performance graph and real single-frame image test captured using the ZWO Ha filter.
Filter Transmission Curve Graph
Real Test (ASI1600 Mono, 300s Frame)
| Brand | ZWO |