Sources & Sensors
The signals we wish to record in astro-imaging (sources) are star light and light excited by stellar radiation in gas clouds (nebulae). The major elements in the stars and the gas clouds are hydrogen (H) and helium (He). Other important elements include oxygen (O), nitrogen (N), and sulfur (S). The wavelengths of the light emitted when these elements are heated to high temperature or excited by radiation of any sort are given on the Emission Lines page.
The signals received by our sensors (eyes, film, CCDs) are a mixture of the star- and nebular-light plus light pollution. The polluting light signals come primarily from manmade sources (street lights, etc) and from atmospheric sources such as gegenschein (a glow arising from sunlight scattered by dust particles in the atmosphere). Fortunately, some of the man-made polluting light can be rejected by filters placed between the source and the sensor. Several types and brands of light pollution-reduction filters are available, and have been used in the astrophotos shown on this web site. The characteristics of these filters are shown in spectra distributed by the manufacturers, eg, Hutech (http://http://www.sciencecenter.net/hutech/tokai/filtplt.htm). Unfortunately, Lumicon (http://www.lumicon.com/) does not publish the spectral response of their filters on their website, but it is supplied with their filters and in their catalog.
The sensors used in astro-imaging are photographic emulsion-based films and charge-coupled devices (CCDs).