Description
Tracking drift occurs when the mount does not exactly compensate for Earth's rotation during an exposure: the sky image slowly slides across the sensor, and each star is recorded as a short streak instead of a point.
The signature is very characteristic: all stars are trailed in the same direction and with the same length across the entire field, from center to corners.
The causes are mechanical: imperfect polar alignment (the mount's axis is not aligned with the celestial pole), absence of autoguiding on long exposures, rough balancing, or friction in the axes.
This is a showstopper defect that ruins the exposure but is easy to diagnose. It must be distinguished from periodic error (S-shaped modulation), field rotation (arcing stars), and guiding oscillations.
Visual signature
All stars form short straight streaks oriented in the same direction, like a subtle trail.
Key point: the trail length is identical everywhere, at the center and in the corners alike. This is what distinguishes drift from optical defects, which worsen toward the edges.
The elongation grows with exposure time: shorter exposures give rounder stars; longer exposures produce more pronounced streaks.
The drift direction is consistent across all frames in a session (RA or Dec axis depending on the source), and the amplitude depends directly on polar alignment quality and exposure length.
Differential diagnosis
Do not confuse with periodic error: periodic error modulates elongation in a sinusoidal pattern from one frame to the next (S-shaped or arcing stars depending on exposure length), whereas tracking drift is constant and unidirectional.
Distinguish from field rotation: rotation produces arcs centered on a point (zero at the rotation center, increasing with distance from it), whereas drift produces identical streaks everywhere.
Separate from guiding oscillations: these produce kidney-bean or short zigzag stars, a sign of over-correction in the guiding loop, not a steady slide.
Do not mistake for wind or vibrations: these deform stars violently and randomly on a few frames, not in a regular and progressive manner.
Distinguish from a poor stack alignment: drift elongates stars on each individual frame, whereas a failed registration only doubles them after stacking. If the target is slowly drifting out of the frame over the course of the night, see also framing issues.
Probable causes
- Imperfect polar alignment (mount axis misaligned with the celestial pole)
- No autoguiding on long exposures
- Rough mount balancing
- Friction or mechanical play in the axes
- Mount underdimensioned for the payload
- Exposures too long for the available tracking accuracy
Course of action
- Improve polar alignment (SharpCap, drift align, properly calibrated polar scope)
- Set up autoguiding (guide scope or off-axis guider) for exposures beyond 30-60 s
- Balance the mount, slightly east-heavy on the RA axis
- Reduce individual exposure time if tracking accuracy is limited
- Check brake tightness and absence of play in the axes
- Match the payload to the mount's torque capacity
- Minimize wind load (cables, accessories) which aggravates drift
The Doc's advice
Drift is the most fundamental mount issue: if your stars are all trailing in the same direction, the mount is not tracking the sky correctly. Two fronts to address. First, polar alignment: a good alignment (SharpCap, drift align, or a properly calibrated polar scope) dramatically reduces drift, especially when imaging unguided. Second, autoguiding: beyond 30 to 60 seconds of exposure, parallel or off-axis guiding corrects residual drift in real time. Also check balancing (slightly east-heavy) and tighten the clutches. And if you are shooting short exposures and still getting drift, it is the polar alignment, not the guiding.
Think you can see this defect in your image?
Run a diagnosisFrequently asked questions
Why are my stars elongated in the same direction?
This is the signature of tracking drift: the mount does not perfectly compensate for Earth's rotation, and the sky slides across the sensor during the exposure. The fact that the elongation is identical in length and direction across the entire field confirms a mechanical origin (polar alignment or tracking), not an optical one (an optical defect would worsen toward the corners). The usual causes are imperfect polar alignment, no autoguiding on long exposures, or balancing and friction issues. The fix is polar alignment and, when needed, autoguiding.
Is autoguiding always required to avoid drift?
Not always. With a good polar alignment and relatively short exposures, many mounts track well enough to produce round stars without guiding. Autoguiding becomes necessary when lengthening exposures (typically beyond 30 to 60 s depending on focal length) or when seeking precision at long focal lengths. It corrects residual drift and periodic error in real time. In short: polar alignment first, autoguiding second for long exposures.
How do I improve my polar alignment?
Several methods, from simplest to most precise. A calibrated polar scope gives a correct alignment for short exposures. SharpCap Polar Align (with a camera) achieves high accuracy in a few minutes. Drift align (King's method) remains the reference: observe the drift of a star and adjust azimuth then altitude until the drift is eliminated. A careful polar alignment drastically reduces drift and residual field rotation, and relieves the guiding load in declination.
Drift or periodic error: how to tell them apart?
By regularity. Tracking drift is constant and always in the same direction: stars trail identically from one frame to the next. Periodic error oscillates at the cadence of the mount's worm gear: the deformations vary cyclically, with some subs sharp and others S-shaped. If the elongation is stable and unidirectional, it is drift (polar alignment or tracking); if it modulates from one frame to the next, it is periodic error, which autoguiding corrects.