Alt-azimuth Mounts

An alt-azimuth mount is a simple mounting system that allows a telescope to rotate around the vertical axis, to adjust the telescope’s azimuth, and the horizontal axis, to adjust the altitude. See coordinate systems for a full explanation of alt-azimuth coordinates.

The advantages of an alt-azimuth mount are that they are very simple to set up and adjust. However, they cannot so easily be used to track the motion of objects across the sky with the Earth‘s rotation as an equatorial mount (see below), although they can be adapted to track objects using computer-controlled motors.

Equatorial Mounts

An equatorial mount allows a telescope to rotate around the imaginary line between the celestial poles, to adjust the telescope’s right ascension, and the line perpendicular to this axis and perpendicular to the barrel of the telescope, to adjust the declination. See coordinate systems for a full explanation of equatorial coordinates.

The advantage of equatorial mounts is that they can be used to track the motion of objects across the sky with the Earth’s rotation. This makes them ideal for astrophotography, particularly when used with a motorised clock drive.

Equatorial mount are more difficult to set up than alt-azimuth mounts, however, as they need careful alignment in order to work as intended. Equatorial mounts also require a weight to counterbalance the telescope, which can make them unsuitable for large telescopes. With Newtonian telescopes, the movement of an equatorial mount can make the eyepiece of the telescope difficult to access.

Dobsonian Mounts

Dobsonian mounts are a particular type of alt-azimuth mount named after John Dobson, who popularised the design in the 1960s. They are particularly suitable for larger Newtonian telescopes as the mirror is close to the ground and moves as little as possible, adding to stability.

The design consists of a flat “ground board” on which sits a “rocker” box, mounted as a turntable.  Semicircular depressions are cut into the rocker box, in which rest the altitude bearings of the telescope.

A Dobsonian mount is a simple mechanism that can be made easily from materials such as plywood and formica, with teflon covered blocks to ensure that the mechanism rotates smoothly.

Motorised, computer-controlled Dobsonian mounts can be used to track objects with the Earth’s rotation.