Almost always because of what it was told rather than what it can do: the time, the location, the levelling or one of the alignment stars was slightly wrong. The mount then builds a model of the sky that is consistently off.
A GoTo mount does not know where anything is. It builds a model from what you tell it and from the stars you centre, and every failure traces back to one of those inputs.
The inputs it depends on
Date, time and time zone; latitude and longitude; which way is level; and the position of two or three known stars. An error in any of them propagates into everything the mount does afterwards.
The commonest mistake
Centring an alignment star that is not the star the mount asked for. The names are unfamiliar, the sky is dark, and a bright star nearby is easy to mistake for the right one. The mount will accept it without complaint and build a model around it.
The second commonest
Time. A clock a few minutes out rotates the whole sky by a corresponding amount, and a time zone or daylight-saving error rotates it by a great deal more.
What to do when it misses consistently
Re-run the alignment rather than nudging every object by hand. If it misses in a consistent direction, the model is wrong and re-synchronising on a bright known object will usually fix the rest of the evening.
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
Does the mount need to be level?
Most alignment routines assume it is, and use that assumption to build their model. A mount visibly out of level will produce pointing that is worse on one side of the sky than the other.
Why does it get worse across the evening?
Usually because the time or the tracking rate is slightly wrong, so the error grows with elapsed time. Re-synchronising on a known object partway through a session is the quick fix.
Last reviewed 17 September 2026