Guider

Role

The Guider module provides automatic telescope guiding. It acquires a reference star field, measures the field drift in real time and sends timed correction pulses to the mount to hold the pointing.

Required devices

Operating phases

Guiding runs through three sequential phases.

Phase 1 — Initialization

The module acquires a reference image and keeps the detected star list as the reference for the whole guiding session. When guiding is started with a valid stored calibration, only this phase runs before guiding; a fresh reference is taken at the current pointing, so calibration can be reused after a slew.

Phase 2 — Calibration

The module measures the mount’s response to correction pulses.

1. A fixed series of pulses is sent West, then East, then North, then South (Calibration steps pulses per direction).
2. The per-pulse image displacement of each pass is measured against the previous frame.
3. The pulse rate is taken from the antisymmetric combinations (West − East) / 2 and (North − South) / 2. A one-directional pass is contaminated by the constant background drift (sidereal tracking error, periodic error, polar-alignment drift); the difference of the two opposite passes cancels that drift and yields one clean RA rate and one clean DEC rate. The raw per-direction rates are still reported, for diagnostics.
4. The CCD orientation is derived from both axes (an RA-axis and a DEC-axis estimate, averaged), with the 180° ambiguity resolved against the RA axis.
5. The RA rate is normalised to the equator (rate × cos(calibration DEC)) so it can be transferred to any declination at guide time.

Before the North and before the South series, an optional DEC backlash kick pulse takes up the DEC axis’ mechanical play first, so the timed measurement pulses aren’t partly wasted re-engaging the gears. Its own movement is excluded from the rate calculation.

The calibration result is saved and reused across sessions, as long as the pier side and camera orientation have not changed.

Phase 3 — Guiding

Continuous correction loop:

1. Acquire an image.
2. Track the star field against the reference (see Star tracking below).
3. Compute the drift, projected onto the RA and DEC axes with the calibration orientation.
4. PI control — the correction is aggressivity × drift + integral gain × accumulated drift. The proportional term alone cannot null a constant drift (it settles at drift / aggressivity); the integral term keeps building until the residual is actually zero. The accumulator is clamped (Integral clamp) and frozen for an axis whose pulse is already at maximum.
5. The RA rate is scaled for the current declination: equatorial rate / cos(current DEC).
6. If DEC backlash compensation is enabled and the DEC direction reversed since the last correction, the current compensation estimate is added on top.
7. Correction pulses are sent to the mount, clamped to Min pulse / Max pulse.
8. Statistics are updated (RMS RA, DEC, total).
9. If a compensated reversal pulse was sent on the previous cycle, the drift reduction it actually achieved is compared with the prediction and the compensation estimate is nudged up (undershoot) or down (overshoot), within the configured bounds.

The first frame of a session is measured but not corrected — it only sets the working zero, so a few pixels of post-slew settle don’t trigger a large first pulse.

A transient loss of star correlation just skips the frame (no pulse); guiding is aborted only after Max consecutive lost frames in a row. A hardware watchdog aborts the session if an exposure, pulse, frame reset or star extraction never completes within Hardware timeout.

Star tracking

Between two frames the star field undergoes a near-pure translation (a meridian flip’s 180° rotation is handled separately by the reverse flags). Tracking looks directly for the dominant translation:

1. Every (reference star, current star) pair implies a candidate translation; the true one is the translation the most stars agree on (a voting / RANSAC step).
2. Its inliers are refined with a two-stage gate and iterative sigma-clipping, and the drift is a flux-weighted mean of the residuals — continuous and sub-pixel.

This is immune to a change in the stars’ flux ranking and to stars appearing or disappearing between frames. The relevant tuning is under Star matching in the parameters.

Parameters

Parameters are reached from the module Parameters menu and a set can be saved as a profile.

Devices

Calibration

Guiding

Star matching

DEC backlash compensation

Manual reversals

Disable corrections

Test aids

Calibration values

Live values

Graphs

Drift (scatter)

XY scatter of the drift of each correction relative to the reference position, with tolerance rings.

Guiding (time series)

PHD2-style graph showing, per correction:

Actions