ZWO 31mm SII 7nm Narrowband Filter - UNMOUNTED - Mark II
(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...)
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 or use it with a H-alpha filter to create bicolor images of nebulosities.
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, you might just buy them one-by-one and still be able to use them, i.e. a SII filter could be used together with a H-alpha for bicolor astrophotography for certain deep sky objects, usually nebulosities...
Some amateur astronomers would use narrowband filters 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.
We would recommend to use narrowband filters with fast imaging telescopes with f/8 and faster focal ratio.
Technical Specifications of ZWO Narrowband Filter Set
Size: 31mm
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)
WHICH DIRECTION TO INSTALL THE FILTERS IN?
It is a tricky question as we asked various manufacturers and most of these manufacturers think that it doesn't matter, however the manufacturer of ZWO filters advises that in the case of their latest (mark II) narrowband filters the coated side of the filter should face the telescope. Well, there is nothing to loose to follow their advice, so lets do it. Now, how to find out which side is the coated? Open the case and remove the filter from its pouch and put it on top of the puch as this is most likely the most dustfree element in your surrounding. Now hold an item, i.e. a pen above the filter, close enough, but make sure that it won't touch the filter. If you see a single reflection as if it was a mirror, that's the coated side; that side should face the telescope. If you see a double reflection, that side has an anti-reflection coating only and it should face the camera sensor.
Image below shows a double reflection of the pen, so that side should face the camera:
Image below shows a single reflection of the pen, so that side should face the telescope:
Transmission Curve
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:
