1. Read the pattern
- Fast zig-zag / overshoot both directions → aggressiveness too high or you're chasing the
seeing. Tuning problem. Keep reading.
- Slow one-way drift on top of the noise → polar alignment or flexure (mechanical), not
aggressiveness.
- Sudden spikes → wind, cable snag, or a balance issue.
- Dead spot right after reversals → that's mechanical play, see
reducing mechanical play in the mount.
2. Diagnose
Run the guiding assistant/calibration and read the guide log: separate RA and DEC, look at
whether corrections consistently overshoot (over-aggressive) or lag (under-aggressive), and compare
the residual error to the measured seeing — you cannot guide out atmospheric blur.
3. Calm it down
- Lower aggressiveness and raise minimum-move so the guider stops reacting to every seeing
wobble.
- Match guide exposure to the seeing (often 2–4 s) — too short chases turbulence.
- Re-calibrate near the celestial equator/meridian and confirm the calibration is clean.
- Tighten polar alignment to reduce the standing DEC drift the guider has to correct.
- Rule out differential flexure (guide scope vs imaging scope moving relative to each other) — if
present, an off-axis guider helps; see choosing a guiding setup.
4. Verify
After tuning, the graph should flatten and RMS drop toward (not below) the seeing limit. Confirm over
a full sub, not a 30-second sample.
When it isn't tuning
If it's a clean dead-zone only at reversals, fix the mount play first
(here); a guider can't tune away mechanical backlash.
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How autoguiding works: why long exposures need guiding.
Hardware causing flexure? Choosing a guiding setup.
Common questions
Why does my autoguiding over-correct or oscillate?
Fast zig-zag/overshoot means aggressiveness is too high or you're chasing the seeing; slow one-way drift is polar alignment or flexure; a dead spot only at reversals is mount backlash.
How do I stabilize erratic guiding in PHD2?
Lower aggressiveness and raise minimum-move, match guide exposure to the seeing (often 2–4 s), re-calibrate near the meridian, tighten polar alignment, and rule out differential flexure.