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Four Basic Filters
#25 Red:
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Martian surface detail
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Clouds on Venus
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Blue clouds on Jupiter, Saturn
#58 Green
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Lunar detail
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Jupiter's Red Spot
#15 Deep Yellow
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Contrast of lunar surface
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Polar Caps and desert regions on Mars
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Orange and red features on Jupiter, Saturn
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Cloud detail on Venus
#80A Medium Blue
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Comet tails
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Lunar surface detail
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Martian polar caps and high clouds
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Venus and Mercury in orange twilight
Never Look At The Sun Through A Telescope!!!
OK, with that said, lets look at the exception. There are filters made that block 99.9% of the sunlight from reaching your eye, allowing us to safely view the sun and its wealth of detail. But we need to make sure we use the right one.
Filters that fit over the front of the telescope, blocking light as it enters, are the only type that are safe to use. There are two types made to fit over the front; Mylar and glass.
Mylar is inexpensive but not durable. It can develop holes that let in harmful sunlight, so Mylar must be treated carefully. Use only Mylar made specifically for solar viewing. Never use space-blanket, or other commercial, Mylar.
Glass costs more but is safer and lasts longer. Again, only use a glass filter made for the purpose of viewing the sun through a telescope, and purchase on made Specifically for your telescope.
Filters
Light is made up of many colors of the spectrum. A telescope gathers all of them and concentrates them into a single point that we then magnify with an eyepiece. Sometimes, however, we can improve the image by blocking a certain color, or wavelength of light, and enhancing others. We may also wish to block the intensity of light, such as from the moon or sun, to allow us to view comfortably and safely. Some light is entirely unwelcome, such as the glow in the sky from city lighting. No matter what your needs, there is a filter for every occasion.
Color Filters: Use them on the planets They bring out the subtle details by increasing contrast of the markings on a planet. Use the guide on the right to help select the right filter for your specific need.
Polarizing Filters: Use them on the moon. Usually available in several densities, or as one filter with a variable adjustment, these filters tone down the brightness of our nearest neighbor. A bright moon viewed through a large telescope is painful to look at. This filter is necessary.
Light Pollution Filters: The name says it all. These filters reflect off wavelengths of light that come from incandescent, fluorescent, and mercury-vapor bulbs. They greatly increase the chance of finding nebulous objects from a light-polluted environment.
V-Block filters. Bright objects viewed in a refractor are ringed by a violet fringe of light that degrades the image. (See Refractors) V-Block filters greatly reduce this fringing, thereby improving the resolution in inexpensive refractors.