Guide scale and the imaging-to-guiding ratio
Your guide scale — arcseconds per pixel on the guide camera — comes from the guide camera’s pixel size and the guide scope’s focal length. Older advice insisted the guide scale be close to the imaging scale, but modern multi-star guiding measures position to a fraction of a pixel, so a short guide scope can reliably guide a much longer imaging scope. A guide scale several times coarser than your imaging scale is now perfectly workable.
Setting minimum move, aggression, and guide exposure
The minimum move tells the guiding software to ignore tiny movements that are really just atmospheric seeing rather than genuine mount drift. Set it too low and the mount chases the seeing; too high and real drift goes uncorrected. This calculator gives a sensible starting minimum move from your guide scale, and the PHD2 Guiding Assistant can fine-tune it. Pair it with a guide exposure of two to four seconds to average out seeing, and keep aggression moderate so corrections are smooth rather than jerky.
Common guiding problems and how to fix them
- Oscillation (stars wobbling back and forth): lower aggression or raise the minimum move.
- Slow drift in declination: check polar alignment and balance.
- Sudden spikes: usually wind, cable snags, or a mount hitting a rough patch in the worm gear.
- Good RMS but elongated stars: differential flexure between guide scope and imaging scope — consider an off-axis guider.
As a rule of thumb, aim to keep total RMS error below about half your imaging pixel scale and your stars will stay round.
How autoguider settings are calculated
Good autoguiding keeps your guide star centred so the error never grows large enough to bloat the stars in your imaging field. The right settings depend on two pixel scales — your imaging camera's and your guide camera's — and your mount's guide rate.
Pixel scale
Both scales use the same formula: scale (″/px) = 206.265 × pixel size (µm) × binning ÷ focal length (mm). The imaging scale tells you how finely your photos are sampled; the guide scale tells you how much sky one guide-camera pixel covers.
Guide-to-imaging ratio
A common rule of thumb is to keep the guide scale no more than about 4–5× the imaging scale. Because a guider can typically centroid a star to ~0.1 pixel, a coarser guide scale than that risks the smallest detectable guide correction being larger than one imaging pixel.
Minimum move
The Total Allowable Error (TAE) is the star wobble the guider can tolerate before it shows in the image — a common starting rule is 80% of the imaging scale, converted to guide pixels (TAE″ ÷ guide scale) and floored at 0.1 px. That gives a starting estimate for PHD2's minimum move; PHD2's own default is 0.2 px, and the authoritative value comes from running PHD2's Guiding Assistant, which measures your seeing directly. (PHD2 simply zeroes any correction below the minimum move — it's a straight threshold, not a radius.)
Guide speed & calibration step
Guide speed is guide rate × 15.04″/s (sidereal). PHD2's calibration step is sized so a calibration moves the star ~25 px over your chosen number of steps: step (ms) = 25 × guide scale ÷ (15 × guide rate) ÷ steps × 1000 ÷ cos(dec) — capped at PHD2's 6-step minimum and rounded up to the nearest 50 ms.
Frequently asked questions
What guide scale should I aim for?
Keep your guide camera's pixel scale within roughly 4–5× your imaging scale. Finer (a longer guide scope or smaller guide pixels) is fine; much coarser means the smallest guide correction may exceed one imaging pixel.
What should I set Minimum Move to in PHD2?
This tool gives a starting estimate (≈80% of imaging scale, in guide pixels, floored at 0.1 px). PHD2's built-in default is 0.2 px. For the best value, run PHD2's Guiding Assistant — it measures your seeing and recommends a minimum move directly. Typical values land around 0.1–0.3 guide px.
What is the calibration step for?
It's how long PHD2 pulses the mount during each calibration move. Too small and calibration takes forever or fails; too large and it overshoots. Aim for about 12 steps to reach the target distance — this tool sizes the pulse for that.
Does declination matter?
Yes — RA moves slow down by cos(declination), so PHD2 lengthens the calibration step near the pole. Calibrate near the celestial equator (Dec 0°) and near the meridian for the most reliable result.
What guide exposure should I use?
1–3 seconds is typical. Longer exposures average out seeing and give a steadier centroid, but respond more slowly to mount errors. Match it to your mount's smoothness and the seeing.
Frequently asked questions
What is a good guiding RMS?
A total RMS error below about half your imaging pixel scale generally gives round stars. For many setups that means guiding under roughly 0.5 to 1.0 arcseconds RMS.
How do I set the PHD2 minimum move?
Set minimum move so PHD2 ignores seeing-induced jitter but still corrects real drift. This calculator suggests a starting value from your guide scale; refine it with the PHD2 guiding assistant.
What guide scale do I need?
Your guide scale in arcseconds per pixel comes from your guide camera pixel size and guide scope focal length. A guide scale a few times coarser than your imaging scale is usually fine for accurate guiding.
What is a good guiding-to-imaging ratio?
Modern multi-star guiding works well even when the guide scale is several times the imaging scale, so a short guide scope can guide a long imaging scope thanks to sub-pixel centroiding.