Description of the Parts of a Telescope

Telescope Tube

• The telescope tube houses the telescopic lenses, eyepieces and reflectors or prisms that gather and focus the light beams that allow users to get a closer look at distant objects. Telescope tubes are fabricated from a variety of materials and come in different sizes, but all function basically the same. The basic telescope tube design is a hollow tube with a primary lens at the front and a focusing lens in the back; however, some designs have a primary mirror in the back, an open front and focusing lens on the top of the telescope.

Primary Lens

• The primary lens, also known as the objective lens, located at the front of a refracting telescope gathers light beams and creates an image of an object at a point in front of the eyepiece assembly known as the focal plane. The base level of a refractor telescope uses a two-element lens; the functionality of a single lens is so poor as to provide little benefit for users. More expensive refractor telescopes use a primary lens with three elements to reduce or do away with color dispersion and enhance contrast and sharpness.

  While the refractor telescope has an objective lens in the front of the telescope, a reflector lens uses a mirror lens in the back of the telescope. The primary mirror lens in the rear of a reflector gathers the light beams from objects and reflects the beams back up through the tube to a secondary mirror, which relays the light beams to the focusing lens.

Eyepiece

• As telescope users look through a telescope's eyepiece, they are viewing magnified images in the focal plane of a refractor telescope, or magnified images from the secondary mirror of a reflector telescope. The lens in an eyepiece is a magnifying glass that magnifies images for the viewer. Eyepieces are located in different places on a telescope, with the eyepiece location dependent on the type of telescope. Some refractor telescopes have eyepieces on the rear of telescope, while others have the eyepiece on the side and use a diagonal or secondary mirror to capture the light beams from the objective lens. Reflector telescopes have the eyepiece on the upper side of the telescope, placed on the side of the telescope.

Finder Scope and Focuser

• Most telescopes are equipped with a finder scope. A finder scope is a small telescope located on the side of a telescope that viewers look through to find the objects that they want to view; objects in the finder scope will appear in the telescope view.

  The eyepiece is encased in the focuser. The focuser is the device that moves the eyepiece either backwards or forwards to bring images into focus that are in the focal plane or on the secondary mirror. Most focusers are adjusted by a knob on the side of the eyepiece that when twisted moves the eyepiece.