Color Filters

When you view through your telescope, you are viewing the full range of the visible light spectrum. There are times however when seeing all the light is not necessarily the best thing. Certain features stand out when certain wavelengths of light are eliminated. Viewing Mars through a blue filter brings out details in the planet's atmosphere and clouds. This occurs because most of the planet's dominant red color cannot pass through the filter. A yellow filter will emphasize the presence of dust in the atmosphere. Mars's surface is best in a red or orange filter. An orange filter can enhance subtle differences in the atmospheres of Jupiter or Saturn. A neutral density filter reduces the brilliant glare of the Moon or Venus by simply reducing the surface brightness and total light. When less than about 17 arc seconds in diameter or close to the horizon, the disk of Venus can be nearly impossible to detect because it is lost in its own glare.

Filters come in two types for Schmidt-Cassegrain telescopes. Eyepiece filters screw directly into the eyepiece barrel. Make certain that when you buy eyepieces that they have threaded barrels; some cheaper units may not be threaded. These are simple filters that are easy to use. Eyepiece filters are also useful for tele-extension photography since an eyepiece is always involved. For prime focus photography, you will need a drop-in filter. These filters are inserted into the telescope's visual back before it is connected to the scope. The drop-in filter will also work for visual applications.

A set of six filters can typically be purchased for a modest price from astronomy equipment retailers for as little as $75. If you intend to do both visual and photographic observations, the drop-in set is probably best for you since the filter can be "dropped in" to a visual back, T-adapter or off-axis guider. A typical set will include red, blue, green, orange, yellow and a neutral density filter. If your telescope is a refractor or Newtonian reflector, you can only use eyepiece filters.

Figure 2.8. Color filter set. Photograph by author.

Figure 2.8. Color filter set. Photograph by author.

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