ZWO EAF Electronic Focuser
An electronic autofocuser that fits over the focuser knob and is driven from the ASIAIR. It refocuses as the temperature drifts through the night, which matters more than expected on a fast optical system where the usable focus tolerance is a few tens of microns.
Why focus will not stay put
Focus is not a setting you establish once. Telescope tubes expand and contract as temperature falls through the night, and the optical path length changes with them. A tube that was in focus at nine in the evening will not be at two in the morning.
How much this matters depends on how much focus error the optical system tolerates, and fast systems tolerate very little. The usable focus zone scales with the square of the focal ratio, so an f/5 instrument has roughly a quarter the tolerance of an f/10 one. At the fast end you are working with a few tens of microns, which is comfortably less than a night's thermal drift.
What the EAF does
It replaces the focuser knob with a stepper motor, controlled from the ASIAIR. Two capabilities follow. It can run an autofocus routine, stepping through positions and measuring star sizes to find the best focus without you touching the telescope. And it can refocus automatically during a session, either at intervals or when the temperature has moved by a set amount.
The second is the one that changes results. Refocusing every hour or so keeps the whole night's subs usable rather than accumulating a slow drift that ruins the last third of them.
Fitting it
Installation means removing a focuser knob and fitting the motor bracket, which is straightforward on supported focusers and fiddly on others. It is worth confirming the bracket matches your focuser before buying, since ZWO supply different fittings.
Backlash is worth checking after installation. Any mechanical focuser has some, and the autofocus routine handles it better if it is measured and configured rather than left at a default.
The unglamorous argument for buying one
Focusing manually means touching the telescope, which means vibration, which means waiting for it to settle, several times a night. It is also difficult to do well, because judging the smallest star by eye on a screen is less accurate than measuring it. Automating it removes a source of error and a source of irritation at the same time.
Who it suits
Anyone imaging at f/6 or faster, anyone whose sessions run several hours, and anyone imaging somewhere with a large overnight temperature swing. For short sessions on slow optics it is a convenience rather than a necessity.