A repeating drift introduced once per revolution of the worm gear that drives a mount. It is a property of the gear rather than a fault, and it sets how long an unguided exposure can be.
Every geared mount has it, and a maker who publishes the figure is telling you something a maker who does not publish it is leaving you to discover.
Where it comes from
A worm gear turns a wheel. Any imperfection in the worm repeats once per revolution of the worm, which on most mounts is a matter of minutes. The result is a slow drift one way and then the other, cycling.
Why it decides exposure length
Over a short exposure the drift is smaller than a star image and invisible. Over a long one it smears. The published figure, in arcseconds, is effectively a statement about how long you can expose without guiding.
What reduces it
Better gears, which cost money. Periodic error correction, which records the cycle and replays a compensation. And guiding, which measures the actual error and corrects it as it happens.
What makes it worse
An unbalanced load, which makes the gear fight in one direction and coast in the other, and a mount loaded near its rating, where flexure adds errors that are not periodic at all.
If this page and the manufacturer’s own documentation disagree, follow the documentation. Every aperture, focal length, payload rating, field, eye relief, transmission figure and thread size on this page comes from the maker’s own published figures, with the unit as printed and the source named. None of it is measured here, and no figure is converted into another convention, because a converted number is a different number. And for any payload figure: makers quote it for observing, and photography asks for a considerably wider margin. A tracking error invisible at the eyepiece is a smeared star on a sensor, and the rating usually excludes the counterweights. This page reports the maker’s figure with the use it was quoted for, and derives no photographic capacity from it.
The questions that keep arriving
Can it be removed?
It can be reduced. Some mounts record the error and replay a correction, and guiding removes what remains. Neither makes the gear perfect; both make the result usable.
Does it matter for visual observing?
Barely. The drift is slow and small, and an object simply wanders a little in the field over minutes. It becomes decisive only when a sensor is integrating.
Last reviewed 17 September 2026