Because it is optical, not mechanical. Tracking errors trail every star in the frame the same way; corner-only distortion is spacing, tilt, an uncorrected field, or a lens used wide open.
The shape of the defect tells you which family of cause you are in, and that is most of the diagnosis.
Every star trailed the same way
Tracking or alignment. The whole frame moved during the exposure. Look at the guiding trace, the balance and the polar alignment.
Corners stretched outward or inward, symmetrically
Spacing. The sensor is not at the distance a reducer or flattener demands. Outward and inward are the two directions of the same error, and the fix is a spacer of the right thickness.
One side worse than the other
Tilt. Something in the train is not square — a sagging clamp, an adapter not seated, a focuser drooping under weight. Rotating the camera moves the bad corner with it.
All corners soft but not stretched
An uncorrected field, which is what a fast telescope does without a flattener, or a camera lens used wide open. Stopping down or fitting the corrector is the answer, and neither is a fault.
The questions that keep arriving
How do I tell spacing from tilt?
Spacing is symmetrical — all four corners behave the same, pointing outward or inward. Tilt is worse on one side of the frame than the other, and the pattern rotates if you rotate the camera.
Can guiding fix it?
No. Guiding corrects movement, and this is not movement. It is where the light lands, which is decided by the optics and by the distances between them.
Last reviewed 17 September 2026