Guider

Role

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

Required devices

Device	Role
Guide camera	Image acquisition for the guide field
ST4 interface	Reception of correction pulses — can be the mount or a camera equipped with an ST4 port

Operating phases

Guiding proceeds through three sequential phases.

Phase 1 — Initialization

The module acquires a reference image and identifies the stars in the field. These stars serve as the reference for the entire guiding session.

Phase 2 — Calibration

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

Pulses are sent in four directions (N, S, E, W)
The pixel displacement caused by each pulse series is measured
A pixels/ms conversion factor is computed for each axis
RA and DEC correction directions are detected automatically

Before the North and before the South pulse series, an extra DEC backlash kick pulse is sent first to take up the DEC axis’ mechanical backlash — otherwise the first measurement pulses of each series would be partially wasted re-engaging the gears, making the computed ms/px rate unreliable. Its own resulting movement is discarded, not included in the rate calculation.

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

Phase 3 — Guiding

Continuous correction loop:

Acquire an image
Match stars using triangle matching (see dedicated section)
Compute drift relative to the reference position
Apply aggressivity coefficients
If DEC backlash compensation is enabled and the DEC pulse direction has reversed since the last correction, add the current compensation estimate on top of the calculated pulse
Send correction pulses to the mount
Update statistics (RMS RA, DEC, total)
If a compensated reversal pulse was sent on the previous cycle, compare the drift reduction it actually achieved against what the calculation alone predicted, and nudge the compensation estimate up (undershoot) or down (overshoot), within the configured bounds

DEC backlash compensation

Mount gears have some mechanical play: whenever DEC guiding reverses direction, the first pulses partly take up that play instead of producing real movement, causing a temporary drift spike. Since the true backlash amount varies with pointing position and mechanical load, a fixed compensation value tends to under- or over-correct depending on where the mount is pointing. The adaptive estimate self-tunes instead: each reversal is both a correction and a measurement, so it converges toward whatever backlash currently exists rather than a value fixed once and left to drift out of tune.

Triangle matching algorithm

Star matching between two images relies on the geometry of triangles formed by the stars, which is invariant under translation, rotation, and scaling.

For each triangle:

ratio = surface / perimeter

This ratio acts as a unique fingerprint to identify the same triangles between the reference image and the current image. The drift is then computed as the mean translation across all matched star pairs.

DEC compensation

RA correction pulses are multiplied by cos(mount_DEC) to compensate for meridian convergence at high declinations.

Parameters

Devices

These parameters are accessible from the module Parameters menu. A set of parameters can be saved as a profile.

Parameter	Description
Exposure	Exposure duration in seconds
Gain	Camera gain
Offset	Camera offset

Calibration

Parameter	Description
Pulse duration	Duration in ms of each pulse sent during calibration
Calibration steps	Number of pulses per axis and direction
DEC backlash kick	Extra pulse (ms) sent before the North and before the South pulse series, to take up mechanical backlash first — 0 disables it

Guiding

Parameter	Description
Max pulse	Maximum correction pulse duration (ms)
Min pulse	Minimum correction pulse duration (ms)
RA aggressivity	Fraction of the computed RA correction that is applied (0–100 %)
DEC aggressivity	Fraction of the computed DEC correction that is applied (0–100 %)
RMS threshold	Multiplier of the current RMS beyond which a correction is discarded
Pier-side RA reverse	Enables automatic RA correction reversal when crossing the meridian
Dither (pixels)	Dithering amplitude in pixels

DEC backlash compensation

Parameter	Description
Enable	Adds an adaptive extra pulse whenever the DEC guide direction reverses, to take up mechanical backlash before the real correction takes effect
Current compensation (ms)	Adaptive estimate, adjusted automatically after each DEC direction reversal — set an initial guess if you know it, otherwise leave at 0 and let it learn
Minimum compensation (ms)	Lower bound for the adaptive estimate
Maximum compensation (ms)	Upper bound for the adaptive estimate
Adjustment step (ms)	How much the estimate is nudged up or down after each reversal, based on measured over/undershoot

Manual reversals

Parameter	Description
Reverse RA	Manually reverses the RA correction direction
Reverse DEC	Manually reverses the DEC correction direction

Disable corrections

Parameter	Description
Disable RA+	Disables correction pulses in the positive RA direction
Disable RA−	Disables correction pulses in the negative RA direction
Disable DEC+	Disables correction pulses in the positive DEC direction
Disable DEC−	Disables correction pulses in the negative DEC direction

Live values

Value	Description
Pulse N	Duration of the last North pulse (ms)
Pulse S	Duration of the last South pulse (ms)
Pulse E	Duration of the last East pulse (ms)
Pulse W	Duration of the last West pulse (ms)
RMS RA	Root mean square of the RA drift (pixels)
RMS DEC	Root mean square of the DEC drift (pixels)
RMS total	Combined RA + DEC RMS (pixels)

Graphs

Drift (scatter)

XY scatter plot showing the drift of each correction relative to the reference position.

Guiding (time series)

PHD2-style graph displaying for each correction:

Curve	Description
RA drift	Measured drift in RA (pixels)
DEC drift	Measured drift in DEC (pixels)
RA pulse	RA correction pulse duration (ms)
DEC pulse	DEC correction pulse duration (ms)
SNR	Signal-to-noise ratio of the star detection
RMS	Current total RMS (pixels)

Actions

Action	Description
Calibrate and guide	Runs all three phases: initialization → calibration → guiding
Calibrate	Runs initialization then calibration only
Guide	Starts guiding (uses existing calibration, or calibrates if none)
Abort	Immediately stops the current sequence
Reset calibration	Clears the saved calibration data
Dither	Randomly offsets the guiding position by Dither (pixels)