Equatorial mount

An equatorial mount is a mount for instruments that compensates for Earth's rotation by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras. The advantage of an equatorial mount lies in its ability to allow the instrument attached to it to stay fixed on any celestial object with diurnal motion by driving one axis at a constant speed. Such an arrangement is called a sidereal or clock drive. An equatorial mount is a mount for instruments that compensates for Earth's rotation by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras. The advantage of an equatorial mount lies in its ability to allow the instrument attached to it to stay fixed on any celestial object with diurnal motion by driving one axis at a constant speed. Such an arrangement is called a sidereal or clock drive. In astronomical telescope mounts, the equatorial axis (the right ascension) is paired with a second perpendicular axis of motion (known as the declination). The equatorial axis of the mount is often equipped with a motorized 'clock drive', that rotates that axis one revolution every 23 hours and 56 minutes in exact sync with the apparent diurnal motion of the sky. They may also be equipped with setting circles to allow for the location of objects by their celestial coordinates. Equatorial mounts differ from mechanically simpler altazimuth mounts, which require variable speed motion around both axes to track a fixed object in the sky. Also, for astrophotography, the image does not rotate in the focal plane, as occurs with altazimuth mounts when they are guided to track the target's motion, unless a rotating erector prism or other field-derotator is installed.