There are several different filters which one can purchase in addition to their eyepiece. Each filter provides a different bonus which is not necessary to astrological viewing but can enhance the experience. Filters screw onto the barrel of the eyepiece and provide different viewing options.
Colored filters are glass pieces of various singular colors which can enhance lunar and planetary details. The colors serve to reduce the wavelength distortion that can blur images. For instance a red filter can bring out a Martian surface detail while a green filter enhances Juniper's red spot. These filters are also referred to as planetary filters.
A light-pollution filter fits onto a Schmidt-Cassegrain eyepiece and blocks elements such as mercury vapor and high-pressure sodium but allows deep-space wavelengths, like hydrogen beta and oxygen III, to pass through.
A moon filter is a glass piece with an aluminum cell which lessens the amount of lunar light and allows one to view the moon more comfortably. This filter also accentuates more lunar detail.
Since observing the sun can be dangerous to the eye, there are solar filters available. A solar filter should block approximately 99.9% of the sun's light. This filter fits over an eyepiece's aperture and allows one to view the sun in all its glory without risking blindness or permanent damage to the eye.
For 1.25" eyepiece barrels there is a variable-polarizing filter which acts as a dimmer switch. This allows the light from bright astral bodies to be lessened by up to 40% or as low as 1%. A variable-polarizing filter is made from two polarized pieces of glass mounted in an aluminum cell. The degree of dimness is adjusted by rotating the filter.
Filters are not massively expensive, but can add to a hobbyist budget if they try to obtain them all. Remember that filters aren’t needed unless one is interested in studying objects like the sun, or wish a more focused view of the night sky.
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