Autoguiding (PHD2 & friends)

The "keep that star a perfect dot for 10 minutes" loop. A small guide camera watches a star through a guide scope (or off-axis guider); a guiding app measures its drift each second and pushes corrections back to the mount. Without it, periodic error in the worm gear smears stars into ovals after ~30 s.

For sequencing the imaging camera see Image Acquisition & Capture; for the mount that's being corrected see Mount & Session Control; for polar alignment (where guiding can only fix so much) see Polar Alignment.

The OSS standard

• ★ ★ PHD2 (Push Here Dummy v2) — BSD-style license, Win/macOS/Linux. The universal autoguider. Every session manager (N.I.N.A, Ekos, SGP, Voyager, ASIAIR) drives PHD2 over its TCP/JSON server protocol — even ZWO's closed ASIAIR runs a PHD2 fork inside. Multi-star guiding (Multi-Star Mode tracks 8 stars for sub-pixel resilience), backlash compensation, predictive PEC, ST4 / pulse-guide / mount-direct / OnStep direct, drift align tool, calibration assistant, server protocol for plate solver / dither integration. The 2026 OSS reference; nothing else comes close.
  • https://openphdguiding.org
• MetaGuide — free Win-only alternative; older; uses cross-correlation rather than centroid; niche; reports better seeing-immune guiding for some setups.

Built into the session manager

• ★ Ekos Internal Guider — built into KStars/Ekos; uses the guide camera attached via INDI; works fine but most users still prefer dropping into PHD2 because the metrics dashboard, multi-star mode, and graph history are better.
• N.I.N.A Direct Guider plug-in — N.I.N.A can drive PHD2 (default) or an "internal" lightweight guider; PHD2 is the recommended N.I.N.A path.
• MaxIm DL Guide — paid Windows; works.
• TheSkyX Direct Guide — paid (Bisque ecosystem); works with Paramount / fork-mount.

Guiding hardware essentials

• ★ Guide scope — 30-50 mm aperture, 120-200 mm focal length; ZWO 30/120, SVBony SV165, Orion Mini, William Optics 32/120 are all fine.
• OAG (off-axis guider) — picks off light through the main scope's optical path; required at long focal lengths (>1500 mm) where guide-scope flexure becomes a problem. ZWO OAG-L, Celestron OAG, QHY OAG-S.
• ★ Guide cameras (mono, small pixels, high QE, low read noise):
  • ZWO ASI120MM Mini / ASI220MM Mini / ASI678MM — most popular OSC/mono guide cams; ~$150–$300.
  • QHY 5L-II-M / 5III-462M — equivalents from QHY.
  • Player One Mars-M — competitive newer entrant.
• ST4 cable — old-school 4-wire pulse guiding directly between guide cam and mount autoguide port; deprecated in favor of:
• ★ Pulse-guide via ASCOM/INDI/Alpaca — the mount itself accepts guiding corrections over its serial/USB connection; lower latency and more accurate than ST4. Default on all modern setups.

Polar alignment limits

PHD2 can't fix poor polar alignment. Field rotation around an inaccurate pole introduces star trails at the corners regardless of how perfect your guiding is. See Polar Alignment before blaming the guider.

The PHD2 metrics that matter

• RMS error in arc-seconds (RA + Dec total) — under 1.0" RMS on a HEQ5/EQ6-class mount is achievable; sub-0.5" on a CEM70 / Mach2; sub-0.3" on direct-drive mounts.
• Star mass — keep it stable; sudden drops mean clouds.
• SNR — ≥ 20 ideal; below 8 PHD2 will drop the star.
• Polar align error (drift align tool) — should match or exceed your hardware's pointing accuracy.

Dithering

Random small (1–5 pixel) shifts between subs that move walking-noise / hot pixels around so they average out in stacking. Done by the sequencer, not PHD2 — but PHD2 reports "settle done" back to the sequencer once the guide error is back in budget.

License / pricing

• PHD2 — BSD-style; free; donations welcome.
• MetaGuide — free.
• Ekos / N.I.N.A internal guiders — free.
• MaxIm DL Guide / TheSkyX Direct Guide — bundled with paid suites.

Pick this if…

• Free, cross-platform, default for everything else: PHD2.
• Already on Ekos and want to stay in one app: Ekos Internal Guider, with PHD2 as the fallback.
• You experience seeing-limited guiding and have read about it: try MetaGuide.
• Long focal length (>1500 mm): OAG + ASI678MM + PHD2 multi-star.
• Short focal length (<800 mm): guide scope + ASI120/220MM + PHD2.