The deep-red wavelength of 656nm is particularly interesting because this is where hydrogen atoms glow and emit hydrogen-alpha light in emission nebulas.

Unfortunately, camera manufacturers design their cameras for normal daytime photography. They want the camera to produce color similar to the way people normally see colors. Human vision is not that sensitive at the wavelength of hydrogen alpha, but digital camera sensors are. So camera manufacturers put a special long-wavelength filter in front of the sensor to filter out most of the hydrogen-alpha wavelength and make the camera's response more similar to human color perception.

While they don't filter out all of this red hydrogen-alpha light that is so abundant in emission nebulas, they do filter out most of it. Only about 25 percent passes through in a stock camera. This makes it much harder to take pictures of these beautiful red nebulas. It's not impossible to do it, it's just more difficult.

Amateur astrophotographers have figured out that they can take this long-wavelength filter out, and modify the camera by replacing it with a different filter that passes almost all of this hydrogen-alpha wavelength. This leads us to "modified" cameras.

Several vendors, such as Hap Griffin and Gary Honis, will sell you a modified camera, or they will modify a camera you already have. You can also modify your camera yourself if you are very handy with delicate and tiny electronic and optical parts.

Mouse-Over

Hold your mouse cursor over the image to see a comparison between an image of M8, the Lagoon Nebula, shot with an unmodified DSLR and one shot with a modified camera. The modified camera records much more of the red light of the hydrogen-alpha wavelength.

Above we can see a comparison of the red hydrogen-alpha wavelength recording capabilities of a stock unmodified DSLR camera and a modified DSLR camera in an image of M8, a fairly bright emission nebula. The modified camera's long wavelength filter passes much more hydrogen-alpha light.

Both of these images are single one minute exposures at ISO 1600 shot through a 70mm refractor working at f/4.8 with a 0.8x reducer at 336mm of focal length. Both images were shot as JPEGs in the camera with standard in-camera processing parameters. No other adjustments were made to the images except to crop and re-size them for display here.

The unmodified camera was a Canon 1000D (Digital Rebel XS), and the modified camera was a Canon 20Da.

Considerations

• Modifying your camera voids the manufacturer's warranty!

• If you remove the long-wavelength filter completely and don't replace it with another filter of the same thickness, your autofocus and visual focus through the viewfinder will be incorrect.

• If you remove the long-wavelength filter completely and don't replace it with a filter that filters out ultra-violet and far infra-red, then you will probably suffer from bloated stars in refractors and camera lenses. This is because these types of optical systems do not focus UV and IR at the same place as the normal visual spectrum.

• Removing the long-wavelength filter will cause inaccurate colors in normal daytime photography.

Solutions

• Using a UV-IR replacement filter of the same thickness solves the problems of focus, and star bloat in refractors.

• Using a "hot-mirror" filter in front of your lens, or in the mirror chamber in front of the sensor, with a correctly created custom white balance can restore the camera to a correct color response similar to a stock camera. A "hot-mirror" filter approximates the camera manufacturer's long-wavelength filter and brings the camera's spectral response back to close to what it was before the modification.