Diagnose 05 of 29 Acquisition mechanics

Field rotation

Stars are short arcs curving around one point in the frame, longer toward the edges.

At 100 %: The corners show it most. The arcs all curve the same way around one point, and the stars at that point are sharp.
Causes, most likely first: Alt-az mount without a wedge
An alt-az mount keeps the target centered, but the field turns around it during each sub.
FixKeep subs short enough that the arcs do not show, or put the mount on an equatorial wedge if its maker supports one. Long subs need an equatorial mount.
Rig Reality Check
Polar axis badly misaligned
An equatorial mount whose polar axis is far from the pole rotates the field as well as letting it drift.
FixPolar align with a software routine.
Session Checklist
Looks like, but isn't: Coma
Coma stretches stars straight out from the center. Field rotation curves them around a point.
One frame can't tell you: One frame shows the arcs. It cannot say how far the polar axis is off; a polar alignment routine measures that.
How to confirm the fix: Round stars in the corners of a full-length sub.
Sources: No outside source is cited here. The signature, the causes and their order are the atlas’s own, from imaging experience. The order is a judgment of likelihood; no measured frequency is behind it.

In the symptom picker

Stars are short arcs that curve around one point, longer toward the edgesweight 3