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! Combine it with H-alpha and SII narrowband filters (SHO Set) for tricolour CCD astrophotography or use it with a H-alpha filter to create bicolour images.

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 nebulosity's. Narrowband 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 OIII filter could be used together with a H-alpha for bicolour astrophotography for certain deep sky objects, usually nebulosity's

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 appear 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 OIII Narrowband Filter
Size: 2" filter cell
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: 7.5mm = 5mm + 2.5mm (thread)
Optical Length: 5mm
Thread: M48*0.75 male thread (standard 2" filter thread)

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