The colour of light

Light as we see it, is only a small part of the electromagnetic spectrum.
These filters are mainly used for Planetary imaging, to enhance contrast on the surface of Jupiter, or Saturn. Some are used to aid viewing the Polar Ice-caps on Mars. The one in front, is called a 'Minus Violet' filter, or 'Fringe Killer', and is used to aid correction of poor optics on cheaper refracting telescopes.
One filter I would really love, is a Hydrogen-Alpha (H?) Filter, which allows you to do this.