Description
Periodic error (PE) is a regular oscillation in right-ascension tracking, caused by small imperfections in the mount's worm gear. With each turn of the worm, the star advances and then retreats slightly relative to the ideal motion.
Left uncorrected (in unguided operation), it deforms stars into S shapes, arcs, or broadened streaks depending on the exposure duration relative to the worm period (often a few minutes).
Its most telling signature is the modulation from one frame to the next: some subs are sharp (when the error crosses zero), others are stretched, in a cyclic pattern.
This is a mechanical defect intrinsic to each mount (amplitude varies by product line). Distinct from tracking drift (constant and unidirectional) and from guiding oscillation (over-correction).
Visual signature
Stars take on an S shape, an elongated comma, or an arc, primarily along the right-ascension axis.
The distinguishing sign is the cyclic variation from frame to frame: the deformation follows the period of the worm gear, alternating sharp subs with stretched ones.
On short exposures (shorter than the worm period), you may see only a variable elongation; on long exposures, the full S pattern appears.
As with drift, the effect is uniform across the entire field (mechanical origin), with no aggravation toward the corners.
Differential diagnosis
Not to be confused with tracking drift: drift is constant and always in the same direction (polar alignment), whereas periodic error oscillates and changes from frame to frame at the rate of the worm.
Distinct from guiding oscillation: those come from an autoguider that over-corrects (bean-shaped or zigzag stars), whereas unguided periodic error is the absence of any correction of the mechanical defect.
To be separated from wind and vibration: those are abrupt and random, while periodic error is smooth and cyclic.
Confirmation: the periodicity of the deformation, matching the worm period of the mount, is the signature of periodic error.
Not to be confused with stacking misalignment: periodic error deforms stars on every individual frame, whereas misalignment only doubles them at the stacking stage.
Probable causes
- Worm gear machining imperfections (intrinsic to the mount)
- No autoguider to compensate for the error
- Exposures longer than the worm period
- Backlash in the gear train amplifying the effect
- PEC not recorded or not activated
- Entry-level mount with high periodic error amplitude
Course of action
- Set up autoguiding (corrects the error in real time)
- Record and activate PEC (Periodic Error Correction) for unguided operation
- Shorten exposures to below the threshold where the error becomes visible
- Adjust gear backlash to reduce the added noise
- Ensure good balance for smooth gear engagement
- Sort the subs: keep those taken near the zero-crossing of the error
- Consider a mount with lower periodic error for long focal lengths
The Doc's advice
Periodic error is your mount's mechanics talking: no worm gear is perfect. Three responses, in order of effectiveness. The best: autoguide, which cancels the error in real time regardless of its amplitude -- the universal solution. Otherwise, PEC records and replays the error curve correction, useful in unguided operation but limited. Finally, in pure unguided shooting, shorten exposures to below the duration where the error becomes visible. And before all of that, check for backlash in the gear train: a poorly adjusted worm adds noise on top of the periodic error.
Think you can see this defect in your image?
Run a diagnosisFrequently asked questions
What is the periodic error of a mount?
It is a regular oscillation in right-ascension tracking, caused by mechanical imperfections in the worm gear that drives the axis. With each rotation of the worm (a period of a few minutes), the star advances and then retreats slightly relative to the ideal motion. The amplitude, expressed in peak-to-peak arcseconds, varies with mount quality: from a few arcseconds on high-end mounts to several tens on entry-level ones. Left uncorrected, it deforms stars in unguided operation; autoguiding cancels it.
Does autoguiding correct periodic error?
Yes, completely. In fact, canceling periodic error is the primary purpose of autoguiding: by measuring the position of a guide star and sending real-time corrections, it compensates for periodic error regardless of its amplitude, as well as residual drift. It is the universal solution for long exposures. PEC is a partial alternative in unguided operation (it replays a pre-recorded correction), but it handles neither drift nor non-periodic irregularities. For serious deep-sky imaging, autoguiding remains the reference.
What does PEC (Periodic Error Correction) do?
PEC records the periodic error curve of the worm gear (typically by guiding through one full cycle), then replays it automatically to compensate during subsequent unguided sessions. It is useful for improving tracking without a guide setup, for example in wide-field or lightweight travel configurations. Its limits: it only corrects the strictly periodic component (not drift or sudden jerks), and its effectiveness depends on the repeatability of the mount. On a modern mount, autoguiding delivers better results with fewer adjustments.
Periodic error or tracking drift: how do you tell them apart?
By behavior over time. Periodic error is cyclic: the deformations come and go at the rate of the worm gear, with some frames sharp and others stretched in an S. Tracking drift is constant and unidirectional: all frames trail in the same way, with no modulation. If your stars are consistently elongated in a fixed direction, that is drift (polar alignment); if the elongation modulates from sub to sub, that is the mount's periodic error.