Altazimuth and equatorial mounts

An altazimuth (altitude-azimuth) telescope mount swivels around an axis that points straight up, so that it moves up, down, left, and right. An equatorial mount has its main axis parallel to the axis of the Earth, so that it moves north, south, east, and west. Most fork-mounted telescopes can be set up either way (Figure 3.2). A telescope without a mount is called an optical tube assembly (OTA).

Before telescopes were computerized, equatorial mounts were the only kind that could track the stars. All an equatorial mount has to do is rotate around its polar axis at the right speed, and as far as the telescope is concerned, the stars

Straight up

To celestial pole

(align on Polaris)

Declination

Declination

Right ascension

Polar axis

Wedge

Azimuth

Azimuth

Altitude

Right ascension

Polar axis

Wedge

Angle =

observer's latitude

Altazimuth mount

Equatorial mount

Figure 3.2. An equatorial mount is just an altazimuth mount tilted so that its main axis is parallel to that of the Earth.

stand still; any star that is in the field of view will stay there. Diurnal motion is completely overcome.

A computerized telescope can also track the stars in altazimuth mode, using motors on both axes, controlled by a computer that translates the Earth's rotation into altitude and azimuth. For this to be possible, the computer has to know the position of the celestial sphere relative to the earth, and that requires identifying (or in some cases guessing) the precise positions of two stars.

3.3 Site information

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