You are here


Eyepiece Filters Assortment of telescope eyepiece filtersSeveral different types of filters are available for use with telescope eyepieces. These filters generally work by blocking wavelengths of light that may be interfering with the object you are trying to view. Light pollution filters work by blocking the scattered light from mercury vapor lights and other terrestrial light sources. Colored filters can both block unwanted wavelengths of light and enhance details in an object. For example, observing the planet Mars through a red filter will increase the contrast of the image and make surface details easier to see. A yellow or blue filter will make the Martian polar ice caps more visible. Observing the planet Jupiter through a yellow, blue, or green filter will enhance the details in the cloud bands and make the famous red spot easier to see. These filters are not very expensive. You may want to consider investing in a set of 4 to 6 assorted color filters when your budget can handle it. If you do your observing from areas that are plagued by light pollution, you may want to get yourself a good light pollution filter as well. Solar Filters Solar Filter designed for a Schmidt-Cassegrain telescopeSolar filters are specially designed to allow safe viewing of the Sun. They are typically available in two different styles. The first style is the eyepiece filter. These filters fit directly onto the eyepiece and are typically included with some low-priced telescopes. Because of the intense amount of heat that can build up at the point of focus, these filters are considered to be quite unsafe. They can crack, allowing a sudden rush of light and searing heat into the eye. Eyepiece solar filters should be avoided at all cost! The second style of solar filter is known as the full-aperture filter. These filters fit directly over the aperture of the telescope. Since they block the light at the point where the light initially enters the telescope, they are much safer if used correctly. Full-aperture filters are available in two types. The first type is the optical glass filter. These filters are made from darkened glass that blocks almost all of the light from the Sun. The image rendered by these filters is generally very good. They deliver a natural yellow-orange look to the Sun and provide a comfortable and safe viewing experience. These filters can become quite expensive, however. For large apertures of 6 inches or more, the cost of the filter can easily exceed a hundred dollars. The second type of full-aperture solar filter is the mylar filter. As the name implies, mylar filters are made of reflective sheets of mylar. The advantage of these filters is their low cost. Mylar filters can be obtained for about one third the cost of their optical glass cousins. The disadvantage of mylar filters is that they render an unnatural blue or green image to the Sun. If you are planning on observing and photographing the Sun, purchase the best filter your budget can afford. IMPORTANT: never look directly at the Sun without the use an approved solar filter. Sunlight through a telescope can severely burn your eyes and cause permanent blindness if an approved solar filter is not used.