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 filter sets 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 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. Many amateurs would use a H-alpha filter in combination with an LRGB set for certain objects, mainly nebulosity's.
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
Size: 31mm unmounted
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!
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:
