Alt-Azimuth Telescope Mounts

June 25, 2009

Most telescopes sit on either an alt-azimuth mount or an equatorial mount. Today we cover the former, which is the easiest mount to understand and use. For many visual observers, it’s the only type of mount they will ever need.

The Basics

• Like the name suggests, an alt-azimuth mount moves in two directions: altitude (perpendicular to the horizon), and azimuth (parallel to the horizon). With these two motions, you can point a telescope on such a mount to any object in the sky.

• But an “alt-az” does not follow the natural motion of the sky. Stars and planets appear to move around the sky in circles centered about an imaginary line through the north and south celestial poles. The stars rise and set at an angle to the horizon equal to 90 degrees minus your latitude. And they follow a path in the sky that’s a combination of altitude and azimuth. So to keep an alt-az-mounted scope centered on a celestial object, you’ll have to move the scope in both axes, which is bothersome for visual observing and completely unacceptable for photography.

A “rocker box” altazimuth mount for a Dobsonian telescope

A Deeper Look

• For amateur telescopes, alt-az mounts come in two basic configurations. Long Newtonian reflectors are usually mounted on a fork-type alt-az mount called a “rocker box”. You give them a push in one or both axes to point the telescope. In many cases, the telescope is held in place by the mount’s friction.

• For shorter telescopes, some modern alt-az mounts have two moving joints connected together, like a camera tripod. The telescope is fixed to the mount by means of a saddle that holds a dovetail bar that’s fixed to the telescope. There are locking screws and perhaps controls for fine adjustment of the mount’s position.

• Equatorial mounts, which we’ll cover in a future issue, solve this problem because they follow the path of the stars with the motion of a single axis. Equatorial mounts require fairly precise alignment with the pole, and this takes time. Alt-az mounts require no alignment… just plop them down and you’re ready to go.

Good To Know

Modern electronics and electric motors make it possible for an onboard computer to control the motion of each axis of an alt-azimuth mount to automatically track the motion of the sky. The computer needs to know your latitude and may require the mount to sit on a level surface to accurately track. This is great for visual observing. But it won’t work for long-exposure photography because the field of view undergoes an apparent rotation that will ruin an astrophoto.

Personal View

With little time for visual observing and no time for fancy astrophotography, I only use an alta-zimuth mount. It takes me just minutes to grab the mount and scope and start looking. Superb for one-minute astronomy.

Posted in Telescopes and Observing

Search: