Filters are pieces of glass or plastic that you place in the barrel of an eyepiece to restrict the wavelengths of light that come through in the image. Filters can be used to :
Light pollution filters are valuable for moderate to severely light-polluted skies. They come in broad-band and narrow-band varieties, and can range from $40 to $150. Color and polarizing filters are useful for bringing out details in the moon and planets. They are often available in sets, and can cost from $15 to $30 each. Solar filters are for solar observing, fitting over the end of a telescope to block most of the light from the sun. They are made of aluminum or Mylar, and can cost from $50 to $200.