Filters are widely used in digital astrophotography, particularly with monochrome cameras where images taken in turn through red, green and blue filters allow a colour image to be created in a computer using appropriate software. In this case, the filters are mounted in a filter wheel, which is often motorised and controlled via the computer running the imaging software. For photography using a colour camera, a broad band light rejection filter such as Lumicon’s Deep Sky Filter will reduce sky glow from natural and artificial sources. However, most image processing software will allow the subtraction of light pollution effects so that filters of this type are not as helpful as they were in the days of film photography, and even light pollution gradients can be handled with appropriate software.

A side issue here relates to consumer digital cameras. These have a filter in front of the ccd or cmos detector that stops most of the light from H-alpha emission from being collected. It is possible to remove this filter and, at the time of writing, a Canon SLR camera with the filter removed is available from Hutech in the USA. The modification can also be done in Australia. With an appropriate white point setting, a modified camera can also be used as a normal daylight camera.

All digital cameras used in astrophotography need filtering to cut the amount of UV and IR wavelengths reaching the detector. As the human eye does not respond to these wavelengths, but CCD and CMOS chips do, images that include UV and IR do not reproduce what the eye can, or does, see.

The other use of filters is in narrow-band imaging with a monochrome camera, using filters that let through only a narrow range of wavelengths, usually OIII, H-alpha and SII. This type of imaging has become popular with some astrophotographers who live in light polluted areas, where the filters do not allow the light pollution through. This is an advanced topic and will not be covered here.