In Astrophotography, you often encounter problems that you need to fix in order to take astounding images. Some problems relate to equipment, so you have to get knowledge and find a solution. Some problems relate to processing, which takes a long time to learn properly (and you probably never stop learning). 
But there are some problems that can’t be solved, no matter what you do: weather and light pollution. Nothing more annoying than being unable to pursue a hobby due to bad weather for weeks. 

But at least there is something you can do against light pollution: Imaging in narrowband. 

Of course, the benefit of narrowband isn’t only imaging from a light polluted area, but also higher contrast, more details and overall higher image quality.

Antlia H-alpha Pro filter 3nm

Narrowband filters are special filters that go in front of the camera sensor to only let a specific wavelength of light pass through, meaning you only image objects which emit/reflect that part of the light spectrum. Reflections from inside the earth’s atmosphere (=light pollution) are mostly filtered out. 

There are 2 kinds of filters: “Nebula boosters” like the Optolong dual pass filters, and “true” narrowband filters, which can only be used with a mono camera and a filter wheel (or a filter drawer).

The most common ones of the latter are the following:

Hydrogen-alpha (Ha)

Sulfur II (S II)

Oxygen III (O III)

Filter wheel with L, R, G, B and H-alpha filters

NGC 7000 in SHO.

Not only can you filter out light pollution, you can also create “false color” images. By assigning SHO as the RGB components of an image, you can basically imitate Hubble’s image colors (the well-known golden-light-blue-ish look). 

And because you only let in a very thin part of the light, the resulting images are usually way more detailed and “sharp” than RGB images, and have higher contrast.

The third possibility is adding narrowband data to RGB images. This is usually done to add H-alpha to images of galaxies (seen as the red/purple areas). Doing this requires a lot of knowledge and skill in processing, as it is a very advanced technique.

Should I use narrowband filters?
 
You should consider using narrowband filters if you live in a heavily light polluted area, as they let you cut right through it.
Also, if you are already imaging mono and want to take your images to the next level, I would recommend taking that step. 
 
You need to have a suitable setup, and even more important, a good mount capable of tracking accurately for a long time (at least 3-5 minutes, sometimes up to 20 minutes), as narrowband filters only let in a very small amount of light. In my opinion, narrowband filters are not for beginners. A good first step might be using a dual pass filter, to increase contrast and fight light pollution at least a bit.
If you want to take the path down to narrowband imaging, you will need to learn quite a few new things, but you will also get some breathtaking images like never before.

Written by Johannes Möslein, 29. September 2024