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
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
- Send correction pulses to the mount
- Update statistics (RMS RA, DEC, total)
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 / perimeterThis 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.
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 |
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 |
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) |