ZWO 1.25" H-alpha SII OIII 7nm Narrowband Filter Set
Narrowband filter sets are used to create high contrast deep sky images of certain objects, mainly emission and diffuse nebulae (i.e. Veil Nebula, M42 Orion Nebula, North America Nebula, Horsehead Nebula) or planetary nebulae (i.e. M27 Dumbbell nebula, M57 Ring Nebula, Helix Nebula), just to mention few of well known nebulosities. Narroband filter sets are sometimes used instead of LRGB filter sets and sometimes in combination with them...
Some amateur astronomers would even use them when the Moon is out or if they live in heavily light polluted areas as these filters would practically eliminate the effects of light pollution as a side effect. By applying longer exposure time you will increase the brightness of the nebulosity whilst stars will still apear much fainter than if they were imaged with LRGB filters. The sky's background will also stay darker hence contrast will be increased.
Technical Specifications of ZWO Narrowband Filter Set
Fine-optically polished to ensure accurate 1/4 wavefront over both surfaces
FWHM：7 ± 0.5nm
Thickness of glass: 1.9mm ± 0.03mm
Total Thickness: 9mm = 5.5mm + 3.5mm (thread)
Optical Length: 5.5mm
Thread: M28.5*0.6 male thread (standard 1.25" filter thread)
This filter set contains three filters, H-alpha, OIII and SII. Lets have a look now at these filters one-by-one and find out about their properties.
The ZWO H-Alpha filter has a bandpass of 7nm and passes light at 656nm wavelength. The light transmission rate comes up to appr. 90% (min. 80%). A 7nm type is a very good choice for narrowband H-alpha astrophotography for high-contrast imaging and revealing rich details of a nebula even in areas with strong light pollution, so prepare to have lots of fun with it !
The ZWO SII 7nm narrowband filter passes light at 672nm wavelength with a bandpass of 7nm which is designed for nebula observation and imaging. It is suitable for visual observation on most emission nebulae, planetary nebulae and supernova remnants, use it with H-alpha and OIII narrowband filters (SHO Set) for tricolor CCD astrophotography.
The ZWO OIII 7nm narrowband filter is designed for nebula observation allowing 7nm bandwidth of light centered on a wavelength of 500nm, which corresponds to OIII emission lines, blocking out all other light. Adding the ZWO OIII narrowband filter to your imaging collection will help you go beyond the RGB imaging and will help capture your favorite nebulae in a totally new light, no matter how polluted your skies may be!