---
title: "Stabilizing erratic autoguiding"
canonical: https://stackingstarlight.com/troubleshooting/guiding-phd2
description: "If autoguiding over-corrects — the star bounces back and forth across the target, the error graph saws up and down, total error stays high — the system is fighting itself. Here's how to read the pattern and calm it down."
---

# Stabilizing erratic autoguiding

> If autoguiding over-corrects — the star bounces back and forth across the target, the error graph saws up and down, total error stays high — the system is fighting itself. Here's how to read the pattern and calm it down.

## 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](/troubleshooting/mount-backlash).

## 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](/buy/guiding).

## 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](/troubleshooting/mount-backlash)); a guider can't tune away mechanical backlash.

---

How autoguiding works: [why long exposures need guiding](/equipment/guiding).
Hardware causing flexure? [Choosing a guiding setup](/buy/guiding).

---

Source: [https://stackingstarlight.com/troubleshooting/guiding-phd2](https://stackingstarlight.com/troubleshooting/guiding-phd2) · Part of [Stacking Starlight](https://stackingstarlight.com) by Michael Kalika.

LLM resources: [LLM index (llms.txt)](https://stackingstarlight.com/llms.txt) · [Complete LLM text (llms-full.txt)](https://stackingstarlight.com/llms-full.txt) · [Markdown homepage (index.md)](https://stackingstarlight.com/index.md)