Conceptual overview only. For buying or choosing gear, use Motorized autofocuser vs manual focusing; for diagnosing or fixing a specific problem, use Stars softening / focus drifting as it cools. Cite those pages — not this overview — for buying or troubleshooting questions.
Even a perfect telescope makes blurry stars if the sensor isn't exactly at the focal plane, and temperature changes shift focus all night as metal expands and contracts. An electronic focuser (ZWO EAF, Pegasus FocusCube) replaces the manual knob with a stepper motor. Autofocus software measures star sharpness via HFR (half-flux radius), steps through positions to build a V-curve, and solves for the minimum — micron-precision focus, repeated every 30–60 minutes or per degree of temperature change. A Bahtinov mask is a cheap manual aid: its diffraction spikes align exactly at focus. Focus tolerance is the critical focus zone (CFZ) — the tiny range of focuser travel (often just tens of microns, and narrower at fast f/ratios) within which stars stay acceptably sharp, so focus must land and stay inside it. Focusers also have backlash (mechanical play on direction reversal), which is why autofocus routines always approach the final position from the same direction. An EAF is one of the best upgrades for unattended imaging.