Field Rotation and the Zenith Keyhole

Point an alt-az mount (any smart telescope, a Dobsonian on a tracking platform, a fork mount without a wedge) at a star and track it perfectly: the star stays centered, but the frame slowly pivots around it. That's field rotation, and it isn't a defect — it's geometry. The sky rotates around the celestial pole; an equatorial mount tilts its axis to match, so the frame rides along. An alt-az mount insists on keeping the horizon level, so the sky must turn relative to the sensor.

The rate isn't constant, and knowing where it's fast is the practical skill. It scales with cos(azimuth)/cos(altitude): fastest for targets crossing the meridian high overhead, essentially ZERO for a target sitting due east or due west, and gentler overall from high latitudes. The infamous extreme is the zenith: directly overhead the required rotation becomes instantaneous — which is also why the azimuth motor would need infinite speed there.

The zenith keyhole

Real alt-az trackers give up near the zenith — typically above 80–85° altitude — because the azimuth axis can't slew fast enough. Smart telescopes simply stop or skip. The cruel joke: the zenith is where a target is at its best (least atmosphere), so a target that transits overhead loses its BEST hour to the keyhole, imaged only on the shoulders either side. A target transiting at 60–75° altitude often collects a better night, total, than one that goes straight overhead.

Within each sub, rotation smears corner stars in proportion to the rate and the sensor's diagonal — which is why alt-az devices shoot 10-second subs and stack: short enough that per-sub rotation stays under a pixel almost everywhere in the sky. Across a session, the live stacker derotates between subs and pays with edge crop; frame slightly wide and the crop is free.