# Focusing & Autofocus for Astrophotography

> Critical focus and autofocus: electronic focusers, V-curve/HFR autofocus, temperature drift, the critical focus zone, and Bahtinov masks.

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.

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Source: [https://stackingstarlight.com/equipment/focuser](https://stackingstarlight.com/equipment/focuser) · Part of [Stacking Starlight](https://stackingstarlight.com) by Michael Kalika.

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