The Horsehead Nebula was shot with a hydrogen-alpha filter with a modified DSLR camera.

Narrowband filters pass only a very narrow part of the spectrum, centered on a particular wavelength of interest. Filtering out all of the other wavelengths greatly increases the contrast between the object and the sky background.

Narrowband imaging can allow photos of supernova remnants, planetary nebulas, and certain emission nebulas to be taken on nights when the moon is up since most of the spectrum which brightens the sky is filtered out.

Examples of some wavelengths that you can get narrowband filters for:

• Hydrogen Beta (H-b) 486.1 nm
• OIII (Oxygen III) 500.7 nm
• H-a (Hydrogen-Alpha) 656.3 nm
• SII (Sulphur II) 672.4 nm

The problem with using narrowband filters with DSLR cameras is that they have the Bayer array color filters already in front of the pixels in the sensor. This means that if you are shooting with a red hydrogen-alpha narrowband filter, only 1/4 of the pixels in the camera are going to record any light at all. Worse, stock cameras already have that long-wavelength filter that is blocking most of this red light.

Unfortunately, hydrogen-alpha narrowband filters do not work very well at all with un-modified cameras. However, they will work with a modified camera, even with the Bayer array filters.

Of course, a monochrome dedicated astronomical CCD camera will work much better than a modified DSLR because all of the pixels in the CCD camera will be recording light. But still, a DSLR camera can be used for narrowband imaging, it just takes longer total exposures to get a high signal-to-noise ratio in the final image.

A narrowband filter really produces a monochrome image. The light that passes through a narrowband filter is all of one wavelength and one color. For example, hydrogen-alpha light is red. Oxygen III light is blue-green.

These monochrome filtered images can be used in color images by blending that specific color into a full color image.

Hydrogen-Alpha (H-alpha) Filters

Hydrogen-alpha filters are probably the best known and most popular because the hydrogen-alpha wavelength is responsible the the red color of emission nebulas.

There are a lot of big, bright, and beautiful emission nebulas in the sky, and the hydrogen-alpha wavelength is usually strong and bright in them.

Mouse-Over

A hydrogen-alpha filtered image is blended into a color image here. Hold your mouse cursor over the image to see a comparison between the regular color image and the blended image.

As seen in the image above, it is also possible to use a high-contrast hydrogen-alpha filtered image to enhance the red in a normal color image by using the technique of channel substitution. With this method, the red channel of the hydrogen-alpha image is swapped for the red channel in a separate full color RGB image, or for the L channel in an LRGB color image, or in other combinations.

WARNING! - THIS TYPE OF HYDROGEN-ALPHA FILTER IS NOT MADE FOR SOLAR VIEWING OR PHOTOGRAPHY! NEVER ATTEMPT TO VIEW OR PHOTOGRAPH THE SUN WITHOUT PROPER AND SAFE FILTRATION! INSTANT AND PERMANENT SEVERE EYE DAMAGE COULD OCCUR.

Narrowband Filters - The Bottom Line Narrowband filters filter out all of the other wavelengths of light except the one we are interested in, such as the red light of hydrogen-alpha in emission nebulas. These filters can greatly increase the contrast of an object at these wavelengths. For hydrogen-alpha, you will need a modified camera to use a narrowband filter. For other filters, such as Oxygen III in the blue-green portion of the spectrum, a stock DSLR camera can be used. Narrowband filters don't work as well with DSLR cameras as they do with monochrome CCD cameras, but you can still use them.