This is the sixth of a series of articles about astronomy equipment to help you decide what you might like. If you have questions about a specific piece of equipment, and how it compares to alternatives, send a note to me at and I'll see what I can find out.

The last piece of equipment we need to address is optical filters. Filters selectively allow certain wavelengths of light to pass through while preventing other wavelengths. All filters are used to enhance certain characteristics of an object; the consequence is that they change the actual image from what we would normally see. We will discuss many filters that are commonly available, but there are virtually an infinite number of possible filters.

If you are doing a careful study of planets, a great way to enhance various characteristics is with color filters. Different colors enhance different features. Orange and red filters enhance daytime viewing of Mercury and Venus, dust clouds and surface markings on Mars, and belts, festoons, and polar regions on Jupiter and Saturn. Orange is also the best filter for enhancing lunar features. Yellow filters work similar to orange and red, but on large aperture telescopes it will enhance details on Neptune and Uranus. Green and blue filters will enhance twilight viewing of Mercury's surface details, enhances upper cloud detail on Venus, Mars polar caps, belts on Saturn and Jupiter, and enhances Jupiter's great red spot. A violet filter will greatly enhance the detail in Saturn’s rings and the upper clouds and haze over Mars' polar caps. Different colors enhance different parts of comets as well. Yellow and orange filters work well for viewing the head of the comet. Dust tails are enhanced by yellow, orange and red filters. The gas tails are enhanced by blue filters.

There are also special filters called Swan Filters that are designed specifically for comet viewing. Hydrogen Alpha filters (H-Alpha) are used for viewing flares and prominences on the sun. Solar filters are used to view sunspots. Narrow band filters (more severe) and broad band filters (less severe) are used to improve images when light pollution is a problem. They filter out the light from incandescent, mercury vapor, and sodium lamps. Neutral density and lunar filters reduce the amount of light at all wavelengths. This is very important for viewing close binary stars, the moon, and brighter planets. A polarizing filter will also help improve viewing of bright objects by reducing glare. For viewing deep space objects a green filter will enhance emission nebulae on large telescopes. A Hydrogen Beta (H-Beta) filter will also greatly enhance faint emission nebulae (like the Horse Head and the California) and an Oxygen III (O-III) filter will enhance planetary and diffuse nebulae (like the Ring and the Vail).