Two ways to track the stars

Figure 9.1 shows the two major kinds of telescope mounts, altazimuth and equatorial. Until the 1980s, only an equatorial mount could track the stars; it does so with a single motor that rotates the telescope around the polar axis, which is parallel with the axis of the earth. In order to use an equatorial mount, you have to make the polar axis point in the right direction, a process known as polar alignment and long considered somewhat mysterious, although actually, correct polar alignment can be achieved quickly (see p. 102).

Computerized telescopes can track the stars with an altazimuth mount, or, indeed, a mount whose main axis points in any direction. During setup, the computer has to be told the exact positions of at least two stars. It then calculates how far to move along each axis, moment by moment, to compensate for the earth's rotation.

Altazimuth mounts can track the stars, but they can't stop the field from rotating (Figure 9.2). With an altazimuth mount, "up" and "down" are not the same celestial direction throughout a long exposure. As a result, the image twists around the guide star. Some large altazimuth telescopes include field de-rotators, which are motors that rotate the camera to keep up with the image. More commonly, though, if you're tracking with an altazimuth mount, you simply take short exposures and combine them through a rotate-and-stack operation (p. 110).

Straight up

To celestial pole

(align on Polaris)

Declination s


Polar Axis



Azimuth s





Polar Axis

Wedge h

Angle =

observer's latitude

Altazimuth Mount

Equatorial Mount

Figure 9.1. Two ways to track the stars. The altazimuth mount requires computer-controlled motors on both axes; equatorial needs only one motor, no computer. (From How to Use a Computerized Telescope.)

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