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.
TrackingThe 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.
NPF ruleA formula by Frédéric Michaud (Société astronomique de France) for the longest exposure without star trails on a fixed tripod: t (s) = (35 × N + 30 × p) ÷ f, where N is the f-number, p the pixel size in µm and f the focal length in mm. At f/2.8 with 4 µm pixels and a 24 mm lens it gives about 9 s. It is more reliable than the old 500 rule with today’s sensors.