Should I use an ALT-AZ or equatorial mount?

Telescopes are mounted in two ways: alt-azimuth and equatorial. Both major types allow the telescope to move freely and be pointed around two perpendicular directions called axes (plural for axis). An axis will always be at right angles to the motions it allows. A good example is the axle of a car. It points to the side while letting the wheels rotate and move the car forwards.

The alt-azimuth (alt-az) mount has an altitude axis allowing the scope to be moved in an up and down direction, and an azimuth axis allowing the scope to move from left to right. It is the simpler of the two types. Since it is oriented towards up and down and thus the horizon, it’s very easy to use to point when looking at terrestrial (ground) objects. It is the first choice for spotting scopes and the like used for looking at buildings, landscapes, birds, etc.

However, it’s very difficult to track stars with an alt-az mount because it has to move both up and sideways. Without a computerized mount, there is no easy way to do this.
 

An equatorial mount has a declination (Dec) axis allowing the scope to be moved north and south, and a right ascension (RA) or polar axis allowing the scope to be moved east and west. So far this is similar to the alt-az mount. The key difference is that the polar axis of an equatorially mounted scope must point at or be polar-aligned to the north celestial pole, the point in the sky around which all the other stars appear to rotate. When the telescope's polar axis is pointed at the celestial pole, it is parallel to the Earth's rotational axis. Now the sky’s motion from east to west can be followed easily by turning the polar axis. Since only one motion is needed to track a star’s motion across the sky, this is the choice for astronomical observing and astroimaging. 

It’s awkward to use an equatorial mount for terrestrial objects, as any sweep to the right and left will also have the tube move up and down. Try leveling the polar axis to 90 degrees on the latitude scale. Now the polar (RA) axis acts like the azimuth and the declination axis acts like altitude, making it easy to sweep along the horizon or elevate to look at mountaintops, etc.


Updated 12/25/13