A small camera, on a guide scope or an off-axis guider, watches a star and sends corrections to the mount several times a minute. This makes up for tracking errors and imperfect polar alignment. Exposures of several minutes then keep stars round.
ToolsPolar alignment
Related terms
- 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.
- 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.
- Drift
The slow sliding of a star across the field when tracking is not perfect. Drift in declination betrays imperfect polar alignment; drift in right ascension usually comes from the mount’s speed or gears. In imaging it stretches the stars.
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