A mount with one axis (the right ascension, or polar, axis) parallel to the Earth’s axis of rotation. Once polar aligned, turning that single axis follows the stars, by hand or with a motor. The “German” type, with counterweights, is the most common.
ToolsPolar alignment
Related terms
- Polar alignment
The adjustment that points the polar axis of an equatorial mount at the celestial pole. Its angle above the horizon must equal your latitude (51.5° in London), and it must aim at true north, not magnetic north. Good polar alignment keeps stars from drifting, visually and in photos.
- Counterweight
The weight fixed on a bar opposite the tube on a German equatorial mount. Slide it until the polar axis stays still when unlocked. Good balance spares the motors and improves tracking.
- Tracking
The motion that cancels the Earth’s rotation to keep an object in view. At the sidereal rate the mount makes one turn in 23 h 56 min, about 15 arcseconds per second. At 200× with a 50° eyepiece, an untracked star near the celestial equator crosses the field in about a minute.
- Meridian flip
On a German equatorial mount, the tube can hit the tripod once the object crosses the meridian. The mount then swings the tube over to the other side to keep tracking. In imaging the frame turns by 180° and the object must be re-centred.
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