What are the differences between an Alt-azimuth mount and an equatorial mount?
December 1, 2008
A telescope mount has two functions. First, it should provide a system for smooth controlled movement to point and guide the instrument. Second, it should support the telescope firmly so that you can view and photograph objects without having the image disturbed by movement.
Alt-azimuth mounts (alt-az) and equatorial mounts are the two major types of telescope mounts.
An alt-az mount is the simplest type of mount. It has two motions, altitude (up and down, vertical to horizontal) and azimuth (side-to-side, along the horizon). Good mounts will have slow-motion knobs to make precise adjustments, which aid in keeping tracking motion smooth. These mounts are good for terrestrial observing and for scanning the sky at lower power but are not for deep sky photography. Certain alt-az mounts (GoTo mounts) are now computer-controlled and allow a telescope to track the sky accurately enough for visual use but are still not accurate enough for long exposure photography.
As the Earth rotates around its axis, the stars appear to move across the sky. If you are observing them using an alt-az mount, they will quickly drift out of view. A telescope on an equatorial mount can be aimed at a celestial object and easily track the daily motion, keeping it in your eyepiece. It works by inclining it at an angle equal to your latitude and pointing one axis (called either the polar axis or right ascension (RA) axis) in the same direction as the Earth’s rotational axis (towards the celestial pole). Once the polar axis is parallel to the Earth’s axis and turned at the same rate of speed as the Earth, but in the opposite direction, objects will appear to stand still when viewed through your scope. There is no rotation of the field of view and tracking can be extremely accurate, making the equatorial mount perfect for astrophotography. It has two motions: in RA (east-west) and in declination (north-south).
There are two basic types of equatorial mounts. The German mount is distinguished by its large counterweight opposite the optical tube on the Dec axle. The counterweight is needed to balance the weight of the optical tube. Since the tube position is out on the end of the Dec axis, a German equatorial mount can easily hold large tubes, cameras, guide scopes and other accessories without interfering with the rest of the mount when looking at most parts of the sky. It’s commonly used with refractors and Newtonian reflectors. A fork mount is generally more convenient to use than the German mount. It can track continuously across the meridian, which many German mounts cannot do. Because it has very little or no counterweights, it’s also lighter and more portable. Short optical tubes (catadioptric designs like the Schmidt-Cassegrain) are very often fork-mounted.
Whichever type of mount you choose, it should be set on a tripod or pier-type platform. Bulk and weight are not as important as a good, rigid design that minimizes vibration.