How Telescopes Work

Finders and Other Accessories

Peep sight

Finders are devices used to help aim the telescope at its target, similar to the sights on a rifle. Finders can come in three basic types:

  • peep sights - notches or circles that allow you to line up the target
  • reflex sights - a mirror box that shows the sky and illuminates the target with a red LED diode spot, similar to a laser sight on a gun
  • telescope sight - a small, low magnification (5x to 10x) telescope mounted on the side with a cross hair reticle, like a telescopic sight on a rifle

Some finders come standard on telescopes, while others are sold separately.



Filters are pieces of glass or plastic that you can place in the barrel of an eyepiece to restrict the wavelengths of light that come through in the image.

Set of filters for viewing, including a light pollution filter (left) and colored filters for enhancing contrast in planetary images.

Filters can be used to:

  • enhance the viewing of faint sky objects in light-polluted skies
  • enhance the contrast of fine features and details on the moon and planets
  • safely view the sun (see Observing the Sun for details)
The filter screws into the barrel of the eyepiece.
Completed filter/eyepiece combination.

Dew Caps

Because you will observe during the night, when it may be cool, moisture can condense in your telescope and on the optics. To prevent this, you can use a dew shield, which wraps around the front end of the telescope. The shield extends the length of the tube and allows moisture to condense on the inside of the shield rather than in the tube. Some shields can be heated to prevent moisture from condensing at all.

Other Detectors

Your eye is the principal light detector for any telescope. For most amateur astronomers, this is the only detector that they will ever need. You might want to take photographs of what you see, however, and you can do that with conventional lens and film cameras or with CCD devices/digital cameras. Some astronomers use their telescopes to make scientific measurements with photometers (devices to measure the intensity of light) or spectroscopes (devices to measure the wavelengths and intensities of light from an object).