Drift alignmentA very accurate polar alignment method that watches stars drift in declination. You adjust the axis in azimuth with a star near the meridian and the equator, then in altitude with a star low in the east or west. It takes patience but no polar scope.
GuidingA 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.
Polar alignmentThe 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.