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... Production of the 2" version started in the end of 2019, so it has ever been only available in the improved version, therefore we did not add a Mark II sign to the title as there has never been a Mark I.)
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 nebulosity's. Narrowband 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 appear 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
Size: 2" filter cell
Fine-optically polished to ensure accurate 1/4 wavefront over both surfaces
FWHM：7 ± 0.5nm
Thickness of glass: 2mm
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 tricolour CCD astrophotography.
The ZWO OIII 7nm narrowband filter is designed for nebula observation allowing 7nm bandwidth of light centred 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 favourite nebulae in a totally new light, no matter how polluted your skies may be!
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 pouch as this is most likely the most dust free 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
