ZWO 1.25" H-alpha 7nm Narrowband Filter - Mark II
ZWO 1.25" H-alpha 7nm Narrowband Filter - Mark II (2018 version)
(Since mid 2018 ZWO supplies a new, improved version of their narrowband filters. The difference is visible when shooting night sky "landscapes" with very bright stars in the field of view, especially with the H-alpha and SII filters, the difference is very-very subtle when using the new OIII filters...)
Narrowband filters 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 filters are sometimes used instead of LRGB filter sets and sometimes in combination with them...However, if you don't yet want to commit to full narrowband H-S-O imaging, it could be a good start to add just a H-alpha filter to your collection of LRGB filters or you might even use it just on its own as we'll explain below...
Some amateur astronomers would use a H-alpha filter 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 (thus smaller and sharper) than if they were imaged with LRGB filters. The sky's background will also stay darker hence contrast will be increased. Many amateurs would use a H-alpha filter in combination with an LRGB set for certain objects, mainly nebulosities.
Others would use a H-alpha filter with a colour camera. Although it might seem counter productive, but it is a good compromise when you don't want to spend lots of time with post processing, but sometimes you need that extra narrowband data for a deep sky object... best to use it with fast imaging telescopes with f/8 and faster focal ratio.
Technical Specifications of ZWO H-alpha Narrowband Filter
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)
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 optional ZWO narrowband S-II 7nm filter passes light at 672nm wavelength with a bandpass of 7nm which is designed for nebula observation. 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 optional ZWO narrowband O-III 7nm filter is designed for nebula observation allowing the 7nm bandwidth of light centered on a wavelength of 500nm through, which corresponds to OIII emission lines, blocking out all other light. Adding the ZWO O-III Narrowband filter to your imaging collection will help you go beyond the RGB imaging and will help capture your favorite nebulas in a totally new light, no matter how polluted your skies may be!
After installing the filters in the EFW, make sure that the correct side of the narrowband filter faces to telescope and camera.
- Glass Thickness 2.0+/-0.03 mm (1.25”/31mm/36mm)
- Fine-optically polished to ensure accurate 1/4 wavefront over the both surfaces
- About 90% transmission at H-alpha line 656nm (H-Alpha filter )
- About 90% transmission at SII line 672nm (SII filter )
- About 90% transmission at major OIII line 500nm (OIII filter )
- Infrared wavelength 700-1100nm cut-off
- <0.1% transmission of off-band, OD3(Optical Density)
Comparison of New (Mark II) and Old Narrowband filters:
The New (Mark II) Ha filter is based on a new base glass, resulting in less reflection (i.e. less halo around bright stars).
The New (Mark II) SII filter is based on a new base glass, resulting in less reflection (i.e. less halo around bright stars).
The New (Mark II) OIII filter is based on the same base glass, but with a new standard coating, resulting in improved blocking of off-band light.
Here is a sample image of a test comparison of the old and new ZWO narrowband Ha Filters (ASI1600 mono camera, single frame, 300s, DDP in MaximDL).
The difference is subtle, but visible, especially when there are really bright stars in the field of view: