Astronomy Specialist



Filter type
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These new 31mm narrowband filters are not mounted in a cell and they are designed to fit on the ZWO filter wheels. Narrowband filters enhance the contrast of emission and planetary nebulae. These peculiar nebulae emit light only in specific wavelengths (H𝛼; OIII ; SII) and the narrowband filters have the property to reject all light outside these bands.

The passing band or "window" of these filters is 7nm wide. Considering that the visible light spectrum spaces between 380nm and 740nm, a 7nm filter would block 98% of the unwanted light. As a result, the signal to noise ratio between the light of the nebulae (signal) and the noise (moonlight, light pollution, airglow) is highly increased. 

Nebulae have a strong emission on the band of the hydrogen (H𝛼), oxygen (O III) and the sulfur (S II). Having 3 wavelengths in 3 different areas of the visible spectrum, it is possible to recreate a false colour image of a nebula. 

These filters cut off the infrared and ultraviolet wavelength so the IR-UV cut filter is not necessary. 

This design allows to use telescopes and lenses with a focal ratio up to f/2 without causing vignetting.


Passing band

7 ± 0.5nm (FWHM)

Glass thickness

2.0 ± 0.03 mm

Optical flatness

1/4 λ (wavefront) 

H𝛼 transmission

About 90% at 656 nm

O III transmission

About 90% at 500 nm

S II transmission

About 90% at 672 nm

Transmission off-band

<0.1% OD3 (Optical Density)

SKU filter set


SKU H𝛼 filter


SKU O-III filter


SKU S-II filter


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