How Telescopes Work

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Telescope Mounts

Diagram of telescope types and mounts.
Diagram of telescope types and mounts.

Telescopes must be supported by some type of stand, or mount -- otherwise you would have to hold it all of the time. The telescope mount allows you to:

  • keep the telescope steady
  • point the telescope at the stars or other objects (birds)
  • adjust the telescope for the movement of the stars caused by the Earth's rotation
  • free your hands for other activities (focusing, changing eyepieces, note-taking, drawing)

There are two basic types of telescope mounts:

  • Alt-azimuth
  • Equatorial

The alt-azimuth mount has two axes of rotation, a horizontal axis and a vertical axis. To point the telescope at an object, you rotate it along the horizon (azimuth axis) to the object's horizontal position, and then tilt the telescope, along the altitude axis, to the object's vertical position. This type of mount is simple to use, and is most common in inexpensive telescopes. The alt-azimuth mount has two variations:

  • ball and socket - used in two inexpensive rich-field telescopes. It has a ball shaped end that can rotate freely in the socket mount.
  • rocker box - a low center-of-gravity box mount, usually made of plywood, with a horizontal circular base (azimuth axis) and Teflon bearings for the altitude axis. This mount is usually used on Dobsonian telescopes. It provides good support for a heavy telescope, as well as smooth, frictionless motion.

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The movement of an alt-azimuth mount in relation to a star.

Although the alt-azimuth mount is simple and easy to use, it does not properly track the motion of the stars. In trying to follow the motion of a star, the mount produces a "zig-zag" motion, instead of a smooth arc across the sky. The zig-zag motion in the figure above is exaggerated and simplified for purposes of illustration; actually there would be more steps, and each step would be smaller. This makes this type of mount useless for taking photographs of the stars.

The equatorial mount also has two perpendicular axes of rotation -- right ascension and declination. However, instead of being oriented up and down, it is tilted at the same angle as the Earth's axis of rotation. The equatorial mount comes in two varieties:

  • German equatorial mount - shaped like a "T." The long axis of the "T" is aligned with the Earth's pole.
  • Fork mount - a two-pronged fork that sits on a wedge that is aligned with the Earth's pole. The base of the fork is one axis of rotation and the prongs are the other.

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The movement of an equatorial mount in relation to a star.

When properly aligned with the Earth's poles, equatorial mounts can allow the telescope to follow the smooth, arc-like motion of a star across the sky. Also, they can be equipped with:

  • setting circles - allow you to easily locate a star by its celestial coordinates (right ascension, declination)
  • motorized drives - allow you or your computer (laptop, desktop or PDA) to continuously drive the telescope to track a star.

You need an equatorial mount for astrophotography.